Lithographic method

ABSTRACT

A method of patterning lithographic substrates that includes using a free electron laser to generate EUV radiation and delivering the EUV radiation to a lithographic apparatus which projects the EUV radiation onto lithographic substrates. The method further includes reducing fluctuations in the power of EUV radiation delivered to the lithographic substrates by using a feedback-based control loop to monitor the free electron laser and adjust operation of the free electron laser accordingly, and applying variable attenuation to EUV radiation that has been output by the free electron laser in order to further control the power of EUV radiation delivered to the lithographic apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase entry of InternationalPatent Application No. PCT/EP2014/062691, filed 17 Jun. 2014, whichclaims the benefit of U.S. provisional application 61/836,619, which wasfiled on 18 Jun. 2013, and U.S. provisional application 61/889,954,which was filed on 11 Oct. 2013 and U.S. provisional application61/941,332, which was filed on 18 Feb. 2014 and European patentapplication EP14152443, which was filed on 24 Jan. 2014 and Europeanpatent application EP14164037, which was filed on 9 Apr. 2014 andEuropean patent application EP14169803, which was filed on 26 May 2014and European patent application EP14171782, which was filed on 10 Jun.2014, the contents of all of which are incorporated herein by referencein their entireties.

FIELD

The present invention relates to a lithographic method. The presentinvention also relates to other methods and related apparatus.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may for example project a pattern from a patterning device(e.g. a mask) onto a layer of radiation-sensitive material (resist)provided on a substrate.

The wavelength of radiation used by a lithographic apparatus to projecta pattern onto a substrate determines the minimum size of features whichcan be formed on that substrate. A lithographic apparatus which usesextreme ultraviolet (EUV) radiation, being electromagnetic radiationhaving a wavelength within the range 4-20 nm, may be used to formsmaller features on a substrate than a conventional lithographicapparatus (which may for example use electromagnetic radiation with awavelength of 193 nm). A free electron laser may be used to generate EUVradiation for use by a lithographic apparatus.

It may be desirable to control the power of radiation output by a freeelectron laser. It may be desirable to control the power of freeelectron laser generated radiation received by a lithographic apparatus.

It is an object of the present invention to obviate or mitigate at leastone problem associated with the prior art.

SUMMARY

According to an aspect of the invention there is a provided a method ofpatterning lithographic substrates, the method comprising using a freeelectron laser to generate EUV radiation and delivering the EUVradiation to a lithographic apparatus which projects the EUV radiationonto lithographic substrates, wherein the method further comprisesreducing fluctuations in the power of EUV radiation delivered to thelithographic substrates by using a feedback-based control loop tomonitor the free electron laser, and adjust operation of the freeelectron laser accordingly.

Using a feedback-based control loop in this manner to reduce EUV powerfluctuations is advantageous because it improves the consistency of EUVradiation dose delivered to the lithographic substrates. This providesmore consistent exposure of the lithographic substrates.

The feedback-based control loop may monitor the power of EUV radiationoutput by the free electron laser. Additionally or alternatively, thefeedback-based control loop may monitor the current of electrons in thefree electron laser. Additionally or alternatively, the feedback-basedcontrol loop may monitor the power of a laser used to generate electronsused by the free electron laser. These are examples of parameters whichare correlated to the power of the EUV radiation output by the freeelectron laser. Other parameters may be used.

The feedback-based control loop may operate at a frequency of 10 kHz ormore. A target location on a substrate may receive EUV radiation foraround 1 ms (which corresponds with a frequency of 1 kHz). Reducing EUVpower fluctuations using feedback having a frequency of 10 kHz or moreis advantageous because it will smooth out the fluctuations sufficientlyquickly that a target location will receive a desired dose of EUVradiation (e.g. to within a desired tolerance). That is, the total doseof EUV radiation received by a target location is a desired dose becauseEUV radiation power fluctuations during the 1 ms exposure time periodare smoothed out. If, for example, the feedback loop operated at afrequency of 1 kHz or less, then an adjustment of EUV radiation power tocompensate for a fluctuation when illuminating a substrate targetlocation would not take place sufficiently quickly to compensate thedose of EUV radiation received by the target location.

The method may further comprise applying variable attenuation to EUVradiation that has been output by the free electron laser in order tofurther control the power of EUV radiation delivered to the lithographicapparatus.

The lithographic apparatus may be one of a plurality of lithographicapparatus which receives the EUV radiation.

Variable attenuation of the EUV radiation may be independentlycontrollable for each of the lithographic apparatus.

The variable attenuation may be controlled by a second feedback-basedcontrol loop.

The second feedback-based control loop may operate at a frequency of 1kHz or less.

The second feedback-based control loop may use EUV radiation intensityas measured by a sensor located in the lithographic apparatus, thesensor being located before a projection system of the lithographicapparatus.

Additionally or alternatively, the second feedback-based control loopmay use EUV radiation intensity as measured by a sensor located in thelithographic apparatus, the sensor being located after a projectionsystem of the lithographic apparatus.

According to an aspect of the invention there is provided a method ofcontrolling the dose of EUV radiation delivered by a lithographicapparatus to a target location on a lithographic substrate followinggeneration by a free electron laser, the method comprising using firstand second feedback-based control loops to adjust the intensity of EUVradiation incident at the target location, the first feedback-basedcontrol loop having a faster response than the second feedback-basedcontrol loop.

The second feedback-based control loop may be associated with thelithographic apparatus.

The first feedback-based control loop may be associated with the freeelectron laser.

The intensity of EUV radiation delivered by the lithographic apparatusmay be monitored using a sensor located at a substrate supporting table.The intensity may be measured between exposure of target locations bythe lithographic apparatus.

According to an aspect of the invention there is provided a method ofcontrolling generation of EUV using a free electron laser, the methodcomprising monitoring the power of EUV radiation output by the freeelectron laser and controlling the power of EUV radiation using afeedback-based control loop, wherein adjustments of the power of EUVradiation are performed whilst maintaining a substantially constantwavelength of EUV radiation.

The EUV radiation may be used by a plurality of lithographic apparatusto pattern lithographic substrates.

According to an aspect of the invention there is a provided alithographic system comprising a free electron laser configured togenerate EUV radiation and a lithographic apparatus configured toproject the EUV radiation onto lithographic substrates, wherein theapparatus further comprises a feedback-based control loop comprising asensor configured to monitor the free electron laser and a controllerconfigured to receive an output from the sensor and to adjust operationof the free electron laser accordingly.

The feedback-based control loop may advantageously reduce fluctuationsin the power of EUV radiation delivered to the lithographic substrates.

The feedback-based control loop may be configured to monitor the powerof EUV radiation output by the free electron laser. Additionally oralternatively, the feedback-based control loop may be configured tomonitor the current of electrons in the free electron laser.Additionally or alternatively, the feedback-based control loop may beconfigured to monitor the power of a laser used to generate electronsused by the free electron laser. These are examples of parameters whichare correlated to the power of the EUV radiation output by the freeelectron laser. Other parameters may be used.

The feedback-based control loop may be configured to operate at afrequency of 10 kHz or more.

The apparatus may further comprise an attenuator configured to applyvariable attenuation to EUV radiation that has been output by the freeelectron laser in order to further control the power of EUV radiationdelivered to the lithographic apparatus.

The lithographic apparatus may be one of a plurality of lithographicapparatus which receives the EUV radiation.

The attenuators may be independently controllable for each of thelithographic apparatus.

The variable attenuation may be controlled by a second feedback-basedcontrol loop.

The second feedback-based control loop may operate at a frequency of 1kHz or less

The second feedback-based control loop may comprise a sensor configuredto measure EUV radiation intensity in the lithographic apparatus.

The sensor may be located before a projection system of the lithographicapparatus.

The sensor may be located after a projection system of the lithographicapparatus.

According to an aspect of the invention there is provided an apparatusfor controlling the dose of EUV radiation delivered by a lithographicapparatus to a target location on a lithographic substrate followinggeneration by a free electron laser, the apparatus comprising first andsecond feedback-based control loops operable to adjust the intensity ofEUV radiation incident at the target location, the first feedback-basedcontrol loop having a faster response than the second feedback-basedcontrol loop.

The second feedback-based control loop may be associated with thelithographic apparatus.

The first feedback-based control loop may be associated with the freeelectron laser.

The lithographic apparatus may comprise a sensor located at a substratesupporting table, the sensor being configured to monitor the intensityof EUV radiation delivered by the lithographic apparatus. The intensitymay be measured between exposure of target locations by the lithographicapparatus.

The EUV radiation may be used by a plurality of lithographic apparatusto pattern lithographic substrates.

According to an aspect of the invention there is a provided an injectorfor a free electron laser comprising a photocathode, a radiation sourceoperable to emit a pulsed radiation beam and direct the pulsed radiationbeam to be incident on the photocathode thereby causing the photocathodeto emit a beam of electron bunches which is output from the injector,each electron bunch corresponding to a pulse of the radiation beam, anda control apparatus operable to interrupt the electron beam so as tocause at least one pulse of the radiation beam to have substantially noassociated electron bunch in the electron beam which is output from theinjector.

An advantage associated with controlling the injector in this manner isthat it provides control of the power of a beam of radiation emittedfrom a free electron laser. Control of the free electron laser beampower may be achieved with a reduced effect on other properties of thefree electron laser beam (compared with at least some prior art controlmethods).

The control apparatus may be operable to interrupt the electron beam soas to cause a single pulse of the radiation beam to have substantiallyno associated electron bunch in the electron beam which is output fromthe injector.

The control apparatus may be operable to substantially prevent at leastone pulse of the radiation beam from being incident on the photocathode,thereby interrupting the emission of electron bunches from thephotocathode

The control apparatus may comprise a Pockels cell disposed in the pathof the radiation beam before the radiation beam is incident on thephotocathode and wherein the Pockels cell is switchable between a firstmode of operation in which the Pockels cell is configured to transmitthe radiation beam without changing its polarization state and a secondmode of operation in which the Pockels cell is configured to transmitthe radiation beam and rotate the polarization state of the Pockelscell, and a polarizer disposed between the Pockels cell and thephotocathode and in the path of the radiation beam, wherein thepolarizer is configured to only transmit radiation having a givenpolarization state.

The Pockels cell may comprise an electro-optic crystal, a pair ofelectrodes and a voltage source, the voltage source being operable togenerate a potential difference between the electrodes thereby switchingthe Pockels cell from the first mode of operation to the second mode ofoperation.

The Pockels cell may comprise a plurality of pairs of electrodes and aplurality of voltage sources, each of the plurality of voltage sourcesbeing operable to generate a potential difference between one of theplurality of pairs of electrodes thereby switching the Pockels cell fromthe first mode of operation to the second mode of operation.

The Pockels cell may be configured to rotate the polarization state ofthe radiation beam by around 90° when in the second mode of operation.

The control apparatus may comprise a plurality of Pockels cells disposedin the path of the radiation beam before the radiation beam is incidenton the photocathode and wherein each of the plurality of Pockels cell isswitchable between a first mode of operation in which the Pockels cellis configured to transmit the radiation beam without changing itspolarization state and a second mode of operation in which the Pockelscell is configured to transmit the radiation beam and rotate thepolarization state of the Pockels cell by less than 90° and wherein theplurality of Pockels cells are configured to apply a combined rotationof the polarisation state of the radiation beam of around 90° when eachof the plurality of Pockels cells are in the second mode of operation.

The polarizer may be configured to only transmit radiation having thepolarization state of the radiation beam before the radiation beam isincident on the Pockels cell.

The injector may further comprise a second Pockels cell configured torotate the polarization state of the radiation beam by around 90° whenin the second mode of operation.

The polarizer may be configured to only transmit radiation having apolarization state which is orthogonal to the polarization state of theradiation beam before the radiation beam is incident on the Pockelscell.

The first Pockels cell may be configured to periodically alternatebetween the first mode of operation and the second mode of operationwith a first time period and wherein the second Pockels cell isconfigured to periodically alternate between the first mode of operationand the second mode of operation with the first time period and at aphase difference with respect to the periodic alternation of the firstPockels cell.

The control apparatus may be configured to adjust the phase differencein order to adjust a number of electron bunches which are interrupted ina given time period.

The injector may further comprise a frequency doubling crystal disposedin the path of the radiation beam before the radiation beam is incidenton the photocathode and after the radiation beam has passed through thepolarizer.

The control apparatus may be operable to deflect at least one electronbunch out of the electron beam, thereby interrupting the electron beamwhich is output from the injector.

The control apparatus may comprise a pair of conducting plates disposedeither side of a trajectory of the electron beam, and a voltage sourceoperable to generate a potential difference between the conductingplates, thereby generating a magnetic field between the conductingplates, the magnetic field being sufficient to deflect an electron bunchout of the electron beam.

The injector may further comprise a beam dump arranged to receive anelectron bunch which is deflected out of the electron beam.

According to an aspect of the invention there is provided an injectorfor a free electron laser comprising a photocathode, a radiation sourceoperable to emit a pulsed radiation beam and direct the pulsed radiationbeam to be incident on the photocathode thereby causing the photocathodeto emit a beam of electron bunches which is output from the injector,and a control apparatus operable to defocus at least one bunch of theelectron beam such that at least one defocussed electron bunch in theelectron beam is output from the injector.

The control apparatus may comprise a pair of conducting plates disposedeither side of a trajectory of the electron beam, a voltage sourceoperable to generate a current to flow through the conducting platesthereby generating a magnetic field between the conducting plates, themagnetic field being sufficient to defocus an electron bunch in theelectron beam.

According to an aspect of the invention there is provided a freeelectron laser comprising an injector according to the preceding aspectsof the invention, a particle accelerator operable to accelerate theelectron beam output from the injector, and an undulator operable toguide the accelerated electron beam along a periodic path so as tostimulate emission of a free electron laser radiation beam, wherein thefree electron laser radiation beam comprises a series of pulses, eachpulse corresponding to an electron bunch of the electron beam.

The control apparatus of the injector may be operable to interrupt theelectron beam which is output from the injector, thereby interruptingthe pulses of the free electron laser radiation beam.

The free electron laser may further comprise a controller operable tocontrol the control apparatus of the injector so as to control a numberof pulses of the free electron laser radiation beam which occur in agiven time period.

The undulator may be operable to stimulate emission of an EUV freeelectron laser radiation beam.

The free electron laser may further comprise a detector configured tomonitor the intensity of the radiation beam and output a signal to thecontroller, thereby providing a feedback loop.

According to an aspect of the invention there is provided a lithographicsystem comprising a radiation source comprising a free electron laseraccording to the third aspect of the invention and further comprising alithographic apparatus.

The lithographic apparatus may be arranged to receive a radiation beamcomprising at least a portion of the free electron laser radiation beamwhich is output from free electron laser of the radiation source, thelithographic apparatus may comprise an illumination system configured tocondition the radiation beam received from the radiation source, asupport structure constructed to support a patterning device, thepatterning device being capable of imparting the radiation beam with apattern in its cross-section to form a patterned radiation beam, asubstrate table constructed to hold a substrate, and a projection systemconfigured to project the patterned radiation beam onto a targetlocation of the substrate.

The control apparatus of the injector may be operable to interrupt theelectron beam which is output from the injector, thereby interruptingthe pulses of the free electron laser radiation beam, therebyinterrupting pulses of the radiation beam which is received by thelithographic apparatus, thereby interrupting pulses of the patternedradiation beam which are projected onto a target location of thesubstrate.

The lithographic system may further comprise a controller wherein thecontroller is operable to control the control apparatus of the injectorso as to control a number of pulses of the patterned radiation beamwhich are received by the target location of the substrate in anexposure time period, thereby controlling a dose of radiation which isreceived by the target location of the substrate in the exposure timeperiod.

According to an aspect of the invention there is provided a method ofcontrolling the power of a radiation beam emitted by a free electronlaser, the method comprising directing a pulsed radiation beam onto aphotocathode of an injector and thereby causing the photocathode to emita beam of electron bunches which is output from the injector, eachelectron bunch corresponding to a pulse of the radiation beam,interrupting the electron beam so as to cause at least one pulse of theradiation beam to have substantially no associated electron bunch in theelectron beam which is output from the injector, accelerating theelectron beam using a particle accelerator, and using an undulatoroperable to guide the accelerated electron beam along a periodic path soas to stimulate emission of a free electron laser radiation beam, thepower of the free electron laser beam being reduced by the interruptionof the electron beam.

At least one pulse of the radiation beam may be prevented from beingincident on the photocathode, thereby interrupting the emission ofelectron bunches from the photocathode.

At least one electron bunch may be deflected out of the electron beam,thereby interrupting the electron beam which is output from theinjector.

According to an aspect of the invention there is provided a method ofcontrolling the power of a radiation beam emitted by a free electronlaser, the method comprising directing a pulsed radiation beam onto aphotocathode of an injector and thereby causing the photocathode to emita beam of electron bunches which is output from the injector, eachelectron bunch corresponding to a pulse of the radiation beam,defocusing at least one pulse of the electron beam which is output fromthe injector, accelerating the electron beam using a particleaccelerator, and using an undulator operable to guide the acceleratedelectron beam along a periodic path so as to stimulate emission of afree electron laser radiation beam, the power of the free electron laserbeam being reduced by the at least one defocussed electron beam pulse.

Aspects of the invention may control the dose of EUV radiation deliveredto the wafer stage of lithographic apparatus via controllableinterruption of electron beam current provided by an injector to anaccelerator of a free electron laser. The current interruption may beperiodic. The current interruption may be controlled via feed-backand/or feed-forward systems. Current interruption may provide a currentthat is substantially zero for the duration of interruption.Alternatively, current interruption may be provide a current that isless than approximately 10% of nominal current.

According to an aspect, there is provided a radiation system comprisinga free electron laser, the free electron laser comprising an electronsource operable to produce relativistic electrons and an undulatorcomprising a plurality of magnets, operable to produce a periodicmagnetic field and arranged so as to guide the relativistic electronsalong a periodic path about a central axis such that they interact withradiation in the undulator, stimulating emission of coherent radiation,wherein the undulator is provided with an adjustment mechanism that isoperable, in response to a received signal, to vary one or moreparameters of the undulator such that the irradiance and/or thepolarization of the radiation is altered. The one or more parameters maybe parameters of the free electron laser.

In an embodiment, a lithographic system may be provided including such aradiation system. Such a lithographic system may include one or morelithographic apparatuses.

According to another aspect, there is provided a free electron lasercomprising: an electron source operable to produce relativisticelectrons; and an undulator comprising a plurality of magnets, operableto produce a periodic magnetic field and arranged so as to guide therelativistic electrons along a periodic path about a central axis suchthat they interact with radiation in the undulator, stimulating emissionof coherent radiation, wherein the undulator is provided with anadjustment mechanism that is operable, in response to a received signal,to vary one or more parameters of the undulator such that the irradianceand/or the polarization of the radiation is altered.

The free electron laser may further comprise a radiation sensor operableto determine an irradiance of the emitted radiation and to transmit asignal indicative thereof to the adjustment mechanism.

The undulator may be tapered.

The adjustment mechanism may be operable to alter the magnetic fieldstrength on, or near to, the central axis of the undulator. The magneticfield strength on, or near to, the central axis may be altered by movingthe magnets towards or away from the central axis. In an embodiment, themagnets are moved relative to the central axis in such a way that thepolarization of the radiation remains unaltered. Additionally oralternatively, the magnetic field strength on, or near to, the centralaxis may altered by altering the magnetic field produced by the magnets.

The adjustment mechanism may be operable to alter a period of theundulator.

The periodic path may comprise a helical section.

The free electron laser may further comprise a first deflecting magnetdisposed between the electron source and the undulator, which can be in:an off state wherein the electrons are guided along the periodic path bythe undulator such that they interact with radiation in the undulator,stimulating emission of coherent radiation; or an on state wherein theelectrons are guided along a different path through the undulator suchthat they decouple from radiation in the undulator and substantially noemission of coherent radiation is stimulated.

The free electron laser may further comprise a second deflecting magnetdisposed downstream of the undulator, which is arranged to compensatefor the action of the first deflecting magnet so that electrons exitingthe second deflecting magnet when the first deflecting magnet is in theon state follow substantially the same the trajectory as electronsexiting the second deflecting magnet when the first deflecting magnet isin the off state.

The free electron laser may further comprise a deceleration mechanismfor decelerating electrons and a beam dump for absorbing electrons, thedeceleration mechanism being operable to reduce the energy of electronsthat have left the undulator before they enter the beam dump.

The electron source may comprise a linear accelerator and thedeceleration mechanism may use the linear accelerator to decelerateelectrons that have left the undulator.

At least a part of the deceleration mechanism may be separate from theelectron source.

The deceleration mechanism may comprise an active deceleration mechanismsuch as a synchrotron or a cyclotron.

The deceleration mechanism may comprise a passive deceleration mechanismsuch as a conductive conduit with a rough inner surface which theelectrons pass through.

The undulator may be configured so that the polarization of the emittedradiation is chosen in dependence with one or more mirrors disposedbetween the free electron laser and a patterning device, such that theradiation incident upon the patterning device has a desiredpolarization.

The adjustment mechanism may be operable to alter the polarization ofthe radiation by: splitting the radiation into two components; guidingthe two components along different optical paths; and combining the twocomponents, wherein one of the optical paths comprises a plurality ofreflections such that the polarization vector of the component followingthat optical path is rotated. Additionally or alternatively, forembodiments wherein the undulator comprises a helical undulator, theadjustment mechanism may be operable to alter the polarization of theradiation by adjusting the periodic magnet structure of at least onesection of the undulator relative to other sections. Additionally oralternatively, for embodiments wherein the undulator comprises a planarundulator, the adjustment mechanism may be operable to alter thepolarization of the radiation by altering tapering of at least onesection of the undulator such that a ratio of first and second linearpolarization components is altered. Additionally or alternatively, theadjustment mechanism may be operable to alter the polarization of theradiation by introducing at least one section of planar undulatorsection into the undulator so as to amplify a desired polarizationstate.

Any one or more of the features above (or otherwise herein) may beincluded in any of the following aspects.

According to an aspect, there is provided a lithographic systemcomprising: a free electron laser as described herein; and one or morelithographic apparatuses.

According to an aspect, there is provided a method of producingradiation, the method comprising: producing a beam of relativisticelectrons; using an undulator to guide the beam of relativisticelectrons along a periodically varying path such that it emits aradiation beam; and varying one or more parameters of the undulator inresponse to a received signal, so as to alter the periodic magneticfield and the irradiance of the radiation beam.

According to an aspect, there is provided a free electron lasercomprising: an electron source operable to produce relativisticelectrons; an undulator comprising a plurality of magnets and operableto produce a periodic magnetic field, through which the relativisticelectrons pass; a first deflecting magnet disposed between the electronsource and the undulator, wherein the first deflecting magnet can be in:an off state wherein the electrons are guided along a periodic path bythe undulator such that they emit interact with radiation in theundulator, stimulating emission of coherent radiation; or an on statewherein the electrons are guided along a different path through theundulator such that they decouple from radiation in the undulator andsubstantially no emission of coherent radiation is stimulated.

According to an aspect, there is provided a free electron lasercomprising: an electron source operable to produce relativisticelectrons; an undulator comprising a plurality of magnets and operableto produce a periodic magnetic field and arranged so as to guide therelativistic electrons along a periodic path such that they emitradiation; a beam dump operable to absorb the electrons once they leavethe undulator; and a deceleration mechanism disposed between theundulator and the beam dump, wherein the deceleration mechanism isoperable to reduce the energy of the electrons before they are absorbedby the beam dump, wherein the deceleration mechanism is separate fromthe electron source.

According to an aspect, there is provided a lithographic systemcomprising: a free electron laser operable to emit radiation in a firstdirection; a lithographic apparatus operable to receive the radiationand project it onto a patterning device in a second direction; and oneor more mirrors disposed between the free electron laser and thepatterning device, operable to guide the beam of radiation from thefirst direction to the second direction, wherein the free electron laseris configured so that the polarization of the first radiation beam ischosen in dependence on the one or more mirrors such that thepolarization of the second radiation beam is suitable for irradiatingthe patterning device.

According to an aspect, there is provided an apparatus for convertingthe polarization of a beam of EUV radiation, comprising: two opticalpaths; and a beam splitter operable to receive a beam of EUV radiation,split it into two components and guide each component along differentone of the optical paths, wherein the two optical paths converge andwherein one of the optical paths comprises a plurality of mirrors suchthat the polarization vector of the component following that opticalpath is rotated.

According to an aspect of the invention there is a provided a freeelectron laser, comprising: an electron source for producing an electronbeam comprising a plurality of bunches of relativistic electrons; anundulator arranged to receive the electron beam and guide it along aperiodic path so that the electron beam interacts with radiation withinthe undulator, stimulating emission of radiation and providing aradiation beam; and an adjustable compressor operable to control atleast one of: (i) a charge density distribution of one or more of theplurality of electron bunches along a direction of propagation of theelectron beam before it enters the undulator; or (ii) an average energyof one or more of the plurality of electron bunches before it enters theundulator.

Altering the charge density distribution of the plurality of electronbunches along their propagation direction will alter a gain of theundulator. That is, it will affect the power of the radiation beamoutput by the undulator. The gain of the undulator may be defined as theamount of power output by the undulator as a function of the amount ofpower input into the undulator. The gain of an undulator may bedependent upon a gain length (the distance an electron bunch must travelthrough the undulator for the power of radiation within the undulator toincrease by a factor of e), a length of the undulator, and the amount ofcoupling between the electron bunches and radiation within theundulator. Further, altering the mean energy of the plurality ofelectron bunches will alter the wavelength of the radiation beam (as themean energy increases, the wavelength of the radiation beam decreases).In turn, this will affect the wavelength of the radiation beam output bythe undulator. Therefore, advantageously, the first aspect of theinvention provides a free electron laser whose output power and/orwavelength can be actively controlled.

The adjustable compressor may comprise an adjustment mechanism arrangedto control at least one of: (a) the chirp of one or more of theplurality of electron bunches; or (b) the average energy of electrons inone or more of the plurality of electron bunches.

The adjustment mechanism may comprise a resonant cavity.

The resonant cavity may be arranged such that a phase of the resonantcavity with respect to the electron beam remains substantially constantand the phase is such that an electric field within the cavity issubstantially zero for electrons at a centre of each bunch passingthrough the resonant cavity. Advantageously, since such an arrangementonly adjusts the charge distribution of the electron bunches and doesnot change a mean energy of the electron bunches, the radio frequencypower required to drive the resonant cavity does not depend on theaverage beam current. Therefore, the power required is low and it ispossible to use a less efficient resonant cavity to alter the chirp.

Alternatively, the resonant cavity may be arranged such that a phase ofthe resonant cavity with respect to the electron beam remainssubstantially constant and the phase is such that an electric fieldwithin the cavity is substantially at its maximum or minimum value forelectrons at a centre of each bunch passing through the resonant cavity.

The resonant cavity may be a normally conducting resonant cavity.

Normally conducting resonant cavities such as, for example, coppercavities have relatively low Q values compared to, for example,superconducting cavities that may be used to accelerate the electronbeam. Since the bandwidth of a resonator is inversely proportional toits Q value, the radio frequency power of such a normally conductingcavity can therefore be adjusted with high bandwidth. Advantageously,this allows for significantly faster change of the accelerating fieldgradient within the cavity, as compared to a superconducting radiofrequency cavity. This is beneficial since it allows the output powerand/or wavelength of the free electron laser to be adjusted relativelyquickly.

The adjustable compressor may further comprise a magnetic compressorarranged to compress one or more of the plurality of electron bunchesalong a direction of propagation of the electron beam, the compressionbeing dependent on a chirp of the electron bunch as it enters themagnetic compressor. The magnetic compressor may be disposed downstreamof the adjustment mechanism.

The electron source may comprise a mechanism for producing a bunchedelectron beam and a linear accelerator operable to accelerate thebunched electron beam, the linear accelerator comprising a plurality ofradio frequency cavities, and the adjustable compressor may be separatefrom the linear accelerator.

The free electron laser may further comprise: a controller; and a sensorfor determining a value indicative of a power of the radiation beam, orindicative of a dose of radiation delivered to a target by the radiationbeam, and outputting a signal indicative of the value to the controller,wherein the controller may be operable to vary the charge densitydistribution of one or more of the plurality of electron bunches and/orthe average energy of electrons in one or more of the plurality ofelectron bunches in response to the signal output by the sensor.

According to an aspect of the invention there is provided a lithographicsystem comprising: a free electron laser according to the first aspectof the invention; and at least one lithographic apparatus, each of theat least one lithographic apparatus being arranged to receive at least aportion of at least one radiation beam produced by the free electronlaser.

According to an aspect of the invention there is provided an apparatuscomprising: a radiation source for producing radiation, said radiationsource comprising an adjustment mechanism operable to control awavelength of the radiation; a target location for receipt of theradiation; an optical system arranged to guide the radiation from theradiation source to the target location, the optical system having awavelength dependent transmittance or reflectance; a controller; and asensor for determining a value indicative of a power of the radiation,or indicative of a dose of radiation delivered to the target location bythe radiation, and outputting a signal indicative thereof to thecontroller, wherein the controller is operable to vary the wavelength ofthe radiation using the adjustment mechanism in response to the signaloutput by the sensor.

Advantageously, such an arrangement provides a positive feedback-basedcontrol loop for controlling the dose of radiation supplied by aradiation source.

The radiation source may comprise a free electron laser, which maycomprise: an electron source for producing an electron beam comprising aplurality of bunches of relativistic electrons; and an undulatorarranged to receive the electron beam and guide it along a periodic pathso that the electron beam interacts with radiation within the undulator,stimulating emission of radiation and providing a radiation beam,wherein the adjustment mechanism is operable to vary an average energyof electrons in one or more of the plurality of electron bunches beforeit enters the undulator.

Altering the mean energy of the electron bunches will alter thewavelength of the radiation beam (as the mean energy increases, thewavelength of the radiation beam decreases). In turn, this will affectthe power of the radiation beam output by the undulator and thereforethe dose of radiation supplied by the radiation source to the targetlocation. Further, since the optical system has a wavelength dependenttransmittance or reflectance, altering the wavelength of the radiationbeam will affect the dose of radiation delivered through the opticalsystem to the target location. Therefore, advantageously, such anarrangement provides a free electron laser whose output power andwavelength can be actively controlled. The change in wavelength of theradiation beam may have a greater effect on the dose delivered to thetarget location by the radiation source than the change in the power ofthe radiation beam.

The adjustment mechanism may comprise a resonant cavity.

The resonant cavity may be a normally conducting resonant cavity.

Normally conducting resonant cavities such as, for example, coppercavities have relatively low Q values compared to, for example,superconducting cavities that may be used to accelerate the electronbeam. Since the bandwidth of a resonator is inversely proportional toits Q value, the radio frequency power of such a normally conductingcavity can therefore be adjusted with high bandwidth. Advantageously,this allows for significantly faster change of the accelerating fieldgradient within the cavity, as compared to a superconducting radiofrequency cavity. This is beneficial since it allows the output powerand/or wavelength of the free electron laser to be adjusted relativelyquickly.

The resonant cavity may be arranged such that a phase of the resonantcavity with respect to the arrival of each of the plurality of electronbunches remains substantially constant and the phase is such that anelectric field within the cavity is substantially at its maximum orminimum value for electrons at a centre of each of the plurality ofelectron bunches passing through the resonant cavity.

The electron source may comprise a mechanism for producing a bunchedelectron beam and a linear accelerator operable to accelerate thebunched electron beam, the linear accelerator comprising a plurality ofradio frequency cavities, and the adjustment mechanism may be separatefrom the linear accelerator.

According to an aspect of the invention there is provided a freeelectron laser, comprising: an electron source for producing an electronbeam comprising a plurality of bunches of relativistic electrons, saidelectron beam having a first frequency; an undulator arranged to receivethe electron beam and guide it along a periodic path so that theelectron beam interacts with radiation within the undulator, stimulatingemission of radiation and providing a radiation beam; and an adjustableresonant cavity arranged between the electron source and the undulatorand arranged to operate at a second frequency such that a chirp of theplurality of electron bunches and/or an average energy of the pluralityof electron bunches varies with time.

When the first and second frequencies are different, the resonant cavitywill continuously vary the chirp and mean energy of the plurality ofelectron bunches. The rate of change of the chirp and mean energy isdependent upon difference between the first and second frequencies.Altering the mean energy of electrons in the electron bunches will alterthe wavelength of the radiation beam (as the mean energy increases, thewavelength of the radiation beam decreases). Therefore the fourth aspectof the invention provides a mechanism for increasing the effectivebandwidth of the radiation output by a free electron laser.

The free electron laser may further comprise a magnetic compressorarranged to compress the electron bunch along a direction of propagationof the electron beam, the compression being dependent on a chirp of theelectron bunch as it enters the magnetic compressor.

The resonant cavity may be a normally conducting resonant cavity.

Normally conducting resonant cavities such as, for example, coppercavities have relatively low Q values compared to, for example,superconducting cavities that may be used to accelerate the electronbeam. Since the bandwidth of a resonator is inversely proportional toits Q value, the radio frequency power of such a normally conductingcavity can therefore be adjusted with high bandwidth. Advantageously,this allows for significantly faster change of the accelerating fieldgradient within the cavity, as compared to a superconducting radiofrequency cavity. This is beneficial since it allows the output powerand bandwidth of the free electron laser to be adjusted relativelyquickly.

The electron source may comprise a mechanism for producing a bunchedelectron beam and a linear accelerator operable to accelerate thebunched electron beam, the linear accelerator comprising a plurality ofradio frequency cavities, and the adjustable compressor may be separatefrom the linear accelerator.

The free electron laser may further comprise: a controller; and a sensorfor determining a value indicative of a power of the radiation beam, ora dose of radiation delivered to a target location by the radiationbeam, and outputting a signal indicative thereof to the controller,wherein the controller is operable to control one or more parameters ofthe resonant cavity in response to the signal output by the sensor.

The one or more parameters of the resonant cavity controlled in responseto the signal output by the sensor may comprise an amplitude of anelectric field within the resonant cavity.

The one or more parameters of the resonant cavity controlled in responseto the signal output by the sensor may comprise the second frequency atwhich the resonant cavity operates. This may be achieved by adjustingboth: a frequency of a radio frequency source that supplieselectromagnetic radiation to the resonant cavity; and a geometry of theresonant cavity. The geometry of the resonant cavity may be alteredusing, for example, one or more piezoelectric stretchers and/orcompressors to match a resonant frequency of the resonant cavity to thefrequency of the radio frequency source that supplies electromagneticradiation to the resonant cavity.

According to an aspect of the invention there is provided a lithographicsystem comprising: a free electron laser according to the fourth aspectof the invention; at least one lithographic apparatus, each of the atleast one lithographic apparatus being arranged to receive at least aportion of at least one radiation beam produced by the free electronlaser a controller; and a sensor for determining a value indicative of apower of the radiation, or indicative of a dose of radiation deliveredto a target location within the at least one lithographic apparatus bythe radiation, and outputting a signal indicative thereof to thecontroller, wherein the controller is operable to vary one or moreparameters of the resonant cavity in response to the signal output bythe sensor.

The target location may be, for example, a location on a substrate whichreceives radiation for a period of time during exposure of a substrate.The dose of energy received by the target location may be an integralwith respect to time of a power of the radiation beam over an exposuretime period. For sufficiently long exposure time periods, high frequencyvariations will be averaged out over the exposure time period. This isespecially beneficial for embodiments wherein the dose of radiationreceived by the target location is wavelength dependent and wherein thisdependence strongly depends on the wavelength and bandwidth of theradiation beam. The fifth aspect of the invention allows control overthe bandwidth of the radiation beam and thus allows the bandwidth to beoptimized to reduce dose sensitivity to variation of wavelength.

The one or more parameters of the resonant cavity controlled in responseto the signal output by the sensor may comprise an amplitude of anelectric field within the resonant cavity.

The one or more parameters of the resonant cavity controlled in responseto the signal output by the sensor may comprise the second frequency atwhich the resonant cavity operates. This may be achieved by adjustingboth: a frequency of a radio frequency source that supplieselectromagnetic radiation to the resonant cavity; and a geometry of theresonant cavity. The geometry of the resonant cavity may be alteredusing, for example, one or more piezoelectric stretchers and/orcompressors to match a resonant frequency of the resonant cavity to thefrequency of the radio frequency source that supplies electromagneticradiation to the resonant cavity.

According to an aspect of the invention there is provided a method ofcontrolling a dose of radiation received by a target location,comprising: determining a dependence of a dose of radiation received bya target location on a wavelength and/or power of a radiation beam;producing radiation using an adjustable radiation source operable toproduce radiation of a plurality of different wavelengths; guiding theradiation to a target location via a wavelength dependent opticalsystem; determining a value indicative of a power of the radiation, orindicative of a dose of radiation delivered to the target location bythe radiation; and varying the wavelength of the radiation in dependenceon the determined value so as to control the dose of radiation receivedby the target location.

The step of determining a dependence of a dose of radiation received bya target location on a wavelength and/or power of a radiation beam mayonly be performed once as a calibration step.

According to an aspect of the invention there is provided a freeelectron laser comprising an injector an accelerator and an undulator,the free electron laser being configured to generate an EUV radiationbeam, wherein the undulator comprises undulator modules and one or moredynamic phase shifters, the dynamic phase shifters being operable tochange the power and/or bandwidth and/or spatial power distribution ofthe EUV radiation beam generated by the free electron laser.

The one or more dynamic phase shifters may comprise electromagnetsconfigured to change the length of an electron trajectory when they areactivated.

The one or more dynamic phase shifters may comprise three pairs ofelectromagnets, each pair being provided on opposite sides of theelectron trajectory.

A controller may be operable to selectively supply current to theelectromagnets to activate and switch off the electromagnets.

The controller may be operable to control the size of the currentsupplied to the electromagnets and thereby control the size of the phaseshift applied by the one or more dynamic phase shifters.

A protective tube formed from a conductive material may be providedaround the beam trajectory.

Openings at least partially filled with dielectric may be provided inthe protective tube.

The openings may have tapered ends.

At least part of the protective tube may be formed from conductivematerial having a skin depth which is more than 10 microns but less than1 mm.

The one or more dynamic phase shifters may comprise transverse kickersconfigured to change the length of an electron trajectory when they areactivated.

The one or more dynamic phase shifters may be controlled by acontroller.

The controller may be configured to control the dynamic phase shifterswith a frequency of 10 kHz or more.

According to an aspect of the invention there is provided a freeelectron laser comprising an injector an accelerator and an undulator,the free electron laser being configured to generate an EUV radiationbeam, wherein the undulator comprises undulator modules and two or moredynamic phase shifters, the dynamic phase shifters being operable tochange the bandwidth and/or spatial power distribution of the EUVradiation beam generated by the free electron laser withoutsignificantly changing the power of the EUV radiation beam generated bythe free electron laser.

According to an aspect of the invention there is provided a method ofgenerating an EUV radiation beam using a free electron laser, the methodcomprising using one or more dynamic phase shifters to modify therelative phase between electron movement and the EUV radiation andthereby change the power and/or bandwidth and/or spatial powerdistribution of the EUV radiation beam generated by the free electronlaser.

The one or more dynamic phase shifters may comprise electromagnetsconfigured to change the length of an electron trajectory when they areactivated.

The one or more dynamic phase shifters may comprise three pairs ofelectromagnets, each pair being provided on opposite sides of theelectron trajectory.

Current may be selectively supplied to the electromagnets to activateand switch off the electromagnets.

The size of the current supplied to the electromagnets may becontrolled, thereby controlling the size of the phase shift applied bythe one or more dynamic phase shifters.

The one or more dynamic phase shifters may comprise transverse kickersconfigured to change the length of an electron trajectory when they areactivated.

The dynamic phase shifters may be controlled with a frequency of 10 kHzor more.

According to an aspect of the invention there is provided a method ofgenerating an EUV radiation beam using a free electron laser, the methodcomprising using two or more dynamic phase shifters to modify thebandwidth and/or spatial power distribution of the EUV radiation beamgenerated by the free electron laser without significantly changing thepower of the EUV radiation beam generated by the free electron laser.

According to an aspect of the invention there is a provided ameasurement apparatus for determining a value indicative of a power of aradiation beam, comprising: a sensor; and an optical element forreceiving the radiation beam, the optical element comprising first andsecond regions, the first region arranged to receive a first portion ofthe radiation beam and the second region arranged to receive a secondportion of the radiation beam, the first and second regions formingspatially distinct regions of a surface of the optical element; whereinthe first region is further arranged to form a first branch radiationbeam from the first portion, and direct the first branch radiation beamto the sensor, which is arranged to determine a power of the firstbranch radiation beam, and wherein the second region is further arrangedto form a second branch radiation beam from the second portion, which isnot directed to the sensor.

Advantageously, such an arrangement allows a power of a first portion ofa radiation beam to be determined without requiring a sensor to beplaced in the path of the radiation beam. Therefore, the inventionenables the measurement of the power of radiation beams with very highpowers and intensities, which would otherwise place too high a thermalload on sensors placed directly in their path. In addition, since thesensor need not be placed in the path of the radiation beam, theinvention provides an arrangement wherein there are no limits on thedimensions of the first region of the optical element. In particular,this allows the first region to be sufficiently small that the part ofthe intensity distribution that is used for the power measurement (i.e.the part that contributes to the first branch radiation beam) issignificantly smaller than would be the case if one or more sensors wereplaced in the path of the radiation beam.

The first region may comprise a plurality of spatially separatedsub-regions.

The plurality of spatially separated sub-regions may be distributed overthe optical element.

The plurality of spatially separated sub-regions may form a rectangulartwo dimensional lattice of recesses or protrusions over a surface of theoptical element. The plurality of spatially separated sub-regions mayform other distributions over the surface of the optical element.

The optical element may comprise a grating and a plurality of faces ofthe grating may form the plurality of spatially separated sub-regions.

The first branch radiation beam may be formed by reflection of the firstportion from the first region.

The first region of the optical element may comprise a fluorescentmaterial and the first branch radiation beam may be formed by thefluorescent material absorbing the first portion of the radiation beamand subsequently emitting radiation of a different wavelength whichforms the first branch radiation beam.

The first branch radiation beam may be formed by scattering of the firstportion from the first region.

The optical element may comprise a grazing incidence mirror.

The sensor may comprise an array of sensing elements.

The sensor may comprise a mechanism for converting the wavelength of thefirst branch radiation beam to a longer wavelength before determining apower of the first branch radiation beam.

The sensor may comprise a mechanism for altering a pulse duration of thefirst branch radiation beam.

A dimension of each of the plurality of spatially separated sub-regionsmay be of the order of 100 μm or less.

A dimension of each of the plurality of spatially separated sub-regionsmay be sufficiently small that in the far field the intensitydistribution of the second branch radiation beam is substantially thesame as that of the radiation beam.

A dimension of each of the plurality of spatially separated sub-regionsmay be sufficiently small that disturbance of the shape of a reflectivesurface of the optical element due to thermal expansion distortion inthe proximity of the plurality of spatially separated sub-regions isnegligible.

A dimension of each of the plurality of spatially separated sub-regionsmay be sufficiently small that the total power emitted or scattered by asingle mark is a relatively small fraction of the radiation beam.

The value indicating a power of the radiation beam may be provided to acontrol element. The control element may be arranged to control anaspect of the radiation beam. For example, the control element may bearranged to adjust an intensity distribution, or an average intensity,of the radiation beam. The controller may be arranged to adjust aposition of the radiation beam.

According to an aspect of the invention there is provided a measurementapparatus for determining a value indicative of a power of a radiationbeam, comprising: a sensor; and an optical element for receiving theradiation beam, the optical element being a grating and comprising aplurality of faces, each face being arranged to receive a portion of theradiation beam and form a radiation sub-beam, wherein the radiationsub-beams from the plurality of faces interfere to form: (i) a firstbranch radiation beam, which is directed to the sensor, the sensor beingarranged to determine a power of the first branch radiation beam, and(ii) a second branch radiation beam, which is not directed to thesensor.

According to an aspect of the invention there is provided a radiationbeam steering unit, comprising: a measurement apparatus according to thefirst aspect of the invention; one or more radiation beam steeringmechanisms; and a control unit arranged to receive a signal from thesensor indicative of a power and, in response thereto, to control theone or more radiation beam steering mechanisms to alter the position ofthe radiation beam.

According to an aspect of the invention there is provided a radiationsource, comprising: a mechanism arranged to output a radiation beam; anda measurement apparatus according to the first aspect of the invention,the optical element being arranged to receive the radiation beam.

According to an aspect of the invention there is provided a lithographicsystem, comprising: a radiation source for producing a radiation beam;one or more lithographic apparatuses; and a measurement apparatusaccording to the first aspect of the invention arranged to determine avalue indicative of a power of the radiation beam or a secondaryradiation beam formed therefrom.

According to an aspect of the invention there is provided a lithographicsystem, comprising: a radiation source for producing a radiation beam;one or more lithographic apparatuses; and a radiation beam steering unitaccording to the second aspect of the invention, arranged to steer theradiation beam or a secondary radiation beam formed therefrom.

According to an aspect of the invention there is provided a measurementapparatus for determining a value indicative of a power of a radiationbeam, comprising: a sensor for determining a power of the radiationbeam; and a mechanism for altering a pulse duration of the radiationbeam before the power is determined by the sensor.

Advantageously, the mechanism for altering a pulse duration may simplifypower measurements for radiation beams with relatively short pulses. Forexample, a radiation beam may comprise pulses that are too short to beresolved by known sensing elements, such as fast photo diodes. In suchcases, by increasing the pulse duration, the power can be resolved bysuch sensing elements.

The mechanism for altering a pulse duration of the radiation beam may befurther operable to convert a wavelength of the radiation beam to alonger wavelength before the power is determined by the sensor.

The mechanism for altering a pulse duration of the radiation beam maycomprise a fluorescent material arranged to absorb the radiation beamand subsequently emit radiation of a longer wavelength.

According to an aspect of the invention there is provided a method ofdetermining a value indicative of a power of a radiation beam,comprising: directing the radiation beam towards an optical elementcomprising first and second regions, the first region arranged toreceive a first portion of the radiation beam and the second regionarranged to receive a second portion of the radiation beam, the firstand second regions forming spatially distinct regions of a surface ofthe optical element; forming a first branch radiation beam from thefirst portion; forming a second branch radiation beam from the secondportion; directing the first and second branch radiation beams todifferent locations; and determining a power of the first branchradiation beam.

The first region may comprise a plurality of spatially separatedsub-regions.

The plurality of spatially separated sub-regions may be distributed overthe optical element.

The plurality of spatially separated sub-regions may form a form arectangular two dimensional lattice of recesses or protrusions over asurface of the optical element.

The optical element comprises a grating and a plurality of faces of thegrating may form the plurality of spatially separated sub-regions.

The first branch radiation beam may be formed by reflection of the firstportion from the first region.

The first region of the optical element may comprise a fluorescentmaterial and the first branch radiation beam may be formed by thefluorescent material absorbing the first portion of the radiation beamand subsequently emitting radiation of a different wavelength whichforms the first branch radiation beam.

The first branch radiation beam may be formed by scattering of the firstportion from the first region.

Before the power of the first branch radiation beam is determined, awavelength of the first branch radiation beam may be converted to alonger wavelength.

Before the power of the first branch radiation beam is determined, apulse duration of the first branch radiation beam may be altered.

According to an aspect of the invention there is provided a method ofdetermining a value indicative of a power of a radiation beam,comprising: directing the radiation beam towards an optical elementcomprising a plurality of faces, each face being arranged to receive aportion of the radiation beam and form a radiation sub-beam; formingfirst and second branch radiation beams from the interference betweenthe radiation sub-beams from the plurality of faces; directing the firstand second branch radiation beams to different locations; anddetermining a power of the first branch radiation beam.

According to an aspect of the invention there is provided a method ofsteering a radiation beam, comprising: determining a value indicative ofa power of a radiation beam using the method according to the eighth orninth aspect of the invention; and in response to the determined value,controlling a position of the radiation beam.

For example, the adjusted parameter may be a position of the radiationbeam. The adjusted parameter may be an intensity or an intensitydistribution of the radiation beam. Generally, the parameter to beadjusted may be adjusted by any appropriate means. For example, theadjustment may be carried out by adjusting a source of the radiationbeam, or by adjusting a path of the radiation beam between the source ofthe radiation beam and the optical element.

According to an aspect of the present invention, there is provided anapparatus for adjusting an intensity of radiation used in a lithographicprocess, comprising: a first element for receiving a first radiationbeam and arranged to reflect a portion of the first radiation beam inthe form of a second radiation beam towards a second element, the secondelement being arranged to reflect a portion of the second radiation beamin the form of a third radiation beam away from the second element; andadjustment means adapted to adjust an incidence angle between at leastone of the first radiation beam and the first element and secondradiation beam and the second element so as to vary an intensity of thethird radiation beam.

In this way, the first aspect provides an apparatus for efficientlyadjusting an attenuation of radiation entering the attenuationapparatus, thereby adjusting the intensity of the radiation beam outputfrom the attenuation apparatus. The first aspect provides a mechanismwhich may be implemented in a mechanically efficient and straightforwardmanner, while allowing for rapid adjustments of the intensity of thethird radiation beam.

The third radiation beam may be output from attenuation apparatus, forexample, towards a lithographic apparatus. Alternatively, the thirdradiation beam may be directed towards a further attenuation apparatus.

The incidence angle of the first radiation beam at the first element maybe the same as the incidence angle of the second radiation beam at thesecond element. The apparatus may be arranged to ensure that theincidence angle of the first radiation beam with respect to the firstelement is always substantially the same as the incidence angle of thesecond radiation beam with respect to the second element. In this way,the third radiation beam is reflected from the third element insubstantially the same direction as the direction of propagation of thefirst radiation beam.

The adjustment means may be adapted to adjust the incidence angle of thefirst and second radiation beams between approximately 1 degree andapproximately 10 degrees.

The first element may be arranged to rotate around a first point and/orthe second element arranged to rotate around a second point. Theadjustment means may be arranged to selectively rotate at least one ofthe first and second elements to adjust the incidence angles of thefirst or second radiation beams with the first and second elements. Thisprovides a particularly effective and simple manner of implementing thefirst aspect.

The first element may be arranged to be rotated around the first pointand/or the second element is arranged to be rotated around the secondpoint through an angle of approximately 9 degrees.

The attenuation apparatus may further comprise a third element forreceiving the third radiation beam and for reflecting a portion of thethird radiation beam in the form of a fourth radiation beam and a fourthelement for receiving the fourth radiation beam and for reflecting aportion of the fourth radiation beam in the form of a fifth radiationbeam away from the fourth element.

By provision of the third and fourth elements, an attenuation range ofthe attenuation apparatus may be increased. Alternatively, oradditionally, provision of the third and fourth elements allows for aneffect of reflection by the elements of the attenuation apparatus on apolarity of radiation to be reduced for a given attenuation.

The adjustment means may be adapted to adjust an incidence angle betweenat least one of the third radiation beam and the fourth element and thefourth radiation beam and the fourth element.

The adjustment means may be adapted to adjust the incidence angle of thefirst, second, third and fourth radiation beams with the respectivefirst, second, third and fourth elements between approximately 1 degreesand approximately 5 degrees. In this way, an attenuation range ofbetween approximately 8% and 20% may be achieved while bettermaintaining a polarity of the first radiation beam in the thirdradiation beam.

The first element may be arranged to rotate around a first point, thesecond element arranged to rotate around a second point, the thirdelement arranged to rotate around a third point and the fourth elementarranged to rotate around a fourth point. The adjustment means may bearranged to selectively rotate at least one of the first, second, thirdand fourth elements to adjust the incidence angles of the first, second,third or fourth radiation beams with the respective first, second, thirdor fourth elements.

Each of the first, second, third and fourth elements may be arranged tobe rotated around the respective first, second, third or fourth pointthrough an angle of approximately 4 degrees.

The apparatus may further comprise a controller arranged to control theadjustment means.

The controller may be arranged to receive indications of a radiationintensity from a sensor and to control the adjustment means in responseto said indications. In this way, the attenuation provided by the firstattenuation apparatus may be better controlled. The controller may, forexample, comprise part of a control loop arranged to maintain anintensity of radiation provided at a predetermined location within apredetermined intensity range.

The apparatus may comprise a further attenuation apparatus. The furtherattenuation apparatus may comprise fixed attenuation apparatus. That is,the further attenuation apparatus may provide an attenuation that cannotbe varied, or can be varied only by a small amount compared to thevariation in attenuation achievable using the first and second elements,or using the first to fourth elements. The further attenuation apparatusmay provide an attenuation factor larger than the attenuation of thevariable attenuator. For example, the further attenuation apparatus mayprovide an attenuation factor of ten.

Alternatively, the further attenuation apparatus may comprise adjustableattenuation apparatus. The further attenuation apparatus may beadjustable through a larger range of attenuations than the firstattenuation apparatus, but may be adjustable with a lower frequency thanthe frequency with which the first attenuation apparatus may beadjusted.

The further attenuation apparatus may comprise a chamber containing anEUV absorbing medium, the chamber being arranged in the path of aradiation beam.

The further attenuation apparatus may comprise a pressure sensoroperable to monitor a pressure within the chamber.

The further attenuation apparatus may comprise a gas inlet and a gasoutlet.

The apparatus may further comprise a second controller, wherein thesecond controller is in communication with the pressure monitor and isarranged to control the gas inlet and gas outlet to maintain a pressurewithin the chamber within a predetermined range.

The first and second controller may be the same controller.

The adjustment means may comprise respective adjustment means for eachelement to be adjusted.

The apparatus may further comprise a reflective membrane disposed at anon-normal angle with respect to the direction of propagation of one ofthe radiation beams, wherein the reflective membrane is arranged totransmit a portion of the one of the radiation beams and to reflect aportion of the one of the radiation beams.

The one of the radiation beams may be, for example, the first, second,third, or fourth radiation beams.

According to an aspect of the invention, there is provided alithographic system comprising: a radiation source operable to produce amain radiation beam; an attenuation apparatus according to the firstaspect arranged to receive at least a portion of the main radiationbeam; and at least one lithographic apparatus, the at least onelithographic apparatus being arranged to receive an attenuated radiationbeam from the attenuation apparatus.

For example, the main radiation, or a portion of the main radiation beammay provide the first radiation beam described above with respect to thefirst aspect.

The lithographic system may comprise a beam splitting apparatus arrangedto receive a main radiation beam and output at least one branchradiation beam. The attenuation apparatus may be arranged to receive theat least one branch radiation beam.

The beam splitting apparatus may be arranged to output a plurality ofbranch radiation beams. The lithographic system may comprise arespective attenuation apparatus for each of said plurality of branchradiation beams, each attenuation apparatus arranged to receive arespective one of said plurality of branch radiation beams.

Alternatively, the lithographic system may comprise one or moreattenuation apparatus for some of the plurality of branch radiationbeams. That is, some branch radiation beams may not pass through anattenuation apparatus in the lithographic system.

The radiation source may comprise one or more free electron lasers.

The at least one lithographic apparatus may comprise one or more maskinspection apparatus.

The main radiation beam may comprise EUV radiation.

It will be appreciated that aspects of the present invention can beimplemented in any convenient way including by way of suitable hardwareand/or software. Alternatively, a programmable device may be programmedto implement embodiments of the invention. The invention therefore alsoprovides suitable computer programs for implementing aspects of theinvention. Such computer programs can be carried on suitable carriermedia including tangible carrier media (e.g. hard disks, CD ROMs and soon) and intangible carrier media such as communications signals.

One or more aspects of the invention may be combined with any one ormore other aspects described herein, and/or with any one or morefeatures described in the preceding or following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 is a schematic illustration of a lithographic system according toan embodiment of the invention;

FIG. 2 is a schematic illustration of a lithographic apparatus thatforms part of the lithographic system of FIG. 1;

FIG. 3 is a schematic illustration of a free electron laser that formspart of the lithographic system of FIG. 1;

FIG. 4 is a schematic illustration of an injector which may form part ofthe free electron laser of FIG. 3;

FIG. 5 is a schematic illustration of a first control apparatus whichmay form part of the injector of FIG. 4;

FIG. 6 is a schematic illustration of an alternative embodiment of afirst control apparatus which may form part of the injector of FIG. 4;

FIG. 7 is a schematic representation of a polarization rotation and atransmitted radiation power which may result from operation of the firstcontrol apparatus of FIG. 6;

FIG. 8 is a schematic illustration of a second control apparatus whichmay form part of the injector of FIG. 4;

FIG. 9 schematically depicts a free electron laser according to anembodiment of the invention;

FIG. 10 schematically depicts an apparatus for converting a beam oflinearly polarized radiation to circularly polarized radiation;

FIGS. 11A & 11B schematically depict two different geometries ofconductive piping which may form part of a deceleration mechanism;

FIG. 12 is a schematic illustration of a free electron laser accordingto an embodiment of the invention, which may form part of thelithographic system of FIG. 1;

FIG. 13 is a schematic illustration of a free electron laser accordingto an embodiment of the invention, which may form part of thelithographic system of FIG. 1;

FIG. 14 schematically depicts an undulator of the free electron laseraccording to an embodiment of the invention;

FIG. 15 schematically depicts an embodiment of a dynamic phase shifterwhich may form part of the undulator;

FIG. 16 schematically depicts a protective tube which may form part ofthe undulator;

FIG. 17 schematically depicts an alternative embodiment of a dynamicphase shifter;

FIG. 18 schematically depicts an embodiment of a measurement apparatusaccording to an embodiment of the invention;

FIG. 19 schematically depicts a plan view of an optical element thatforms part of the measurement apparatus of FIG. 18;

FIG. 20 schematically depicts a cross sectional view of an opticalelement that forms part of the measurement apparatus of FIG. 18, throughthe line A-A in FIG. 19;

FIG. 21 schematically depicts part of the measurement apparatus of FIG.18 and a discretely sampled intensity distribution determined by thesensor of the measurement apparatus;

FIG. 22 schematically depicts a cross sectional view of an alternativeembodiment of an optical element that may form part of the measurementapparatus of FIG. 18, through the line A-A in FIG. 19;

FIG. 23 schematically depicts an alternative embodiment of themeasurement apparatus;

FIG. 24 schematically depicts a further alternative embodiment of themeasurement apparatus;

FIG. 25 schematically depicts a further alternative embodiment of themeasurement apparatus;

FIG. 26 schematically depicts a cross sectional view of a section of anoptical element that may form part of the measurement apparatus of FIG.25;

FIG. 27 schematically depicts a further alternative embodiment of themeasurement apparatus;

FIG. 28 is a schematic illustration of an attenuation apparatus of thelithographic system of FIG. 1;

FIG. 29 is a schematic illustration of an alternative attenuationapparatus of the lithographic system of FIG. 1;

FIGS. 30a, 30b are schematic illustrations of further attenuationapparatus of the lithographic system of FIG. 1;

FIGS. 31 and 32 are schematic illustrations of further attenuationapparatus of the lithographic system of FIG. 1; and

FIG. 33 is a schematic illustration of a further attenuation apparatusof the lithographic system of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 shows a lithographic system LS according to one embodiment of theinvention. The lithographic system LS comprises a free electron laserFEL, a beam delivery system and a plurality of lithographic apparatusLAa-LAn (e.g. eight lithographic apparatus). The free electron laser FELis configured to generate an extreme ultraviolet (EUV) radiation beam B(which may be referred to as a main beam). A controller CT controls thepower of EUV radiation emitted from the free electron laser FEL. Asensor apparatus ST monitors the power of the EUV radiation beam outputby the free electron laser beam or a parameter correlated with the powerof the EUV radiation beam. The controller CT adjusts the free electronlaser based on the output of the detector. Thus a feedback-based controlloop is provided, as indicated by the dashed line F1. The sensorapparatus ST may be provided at any suitable location. Apparatus andmethods which may be used to control the output power of the freeelectron laser FEL are described further below. These may for example beused to reduced fluctuations of the power of the EUV radiation beamoutput from the free electron laser (e.g. when averaged over an exposuretime period such as 1 ms). Apparatus and methods which may be used toadjust the output power of the free electron laser whilst maintaining asubstantially constant wavelength are described further below.

The sensor apparatus ST may for example be an EUV radiation detectorwhich is configured to monitor the power of the EUV radiation beam (e.g.by splitting off and measuring a fraction the EUV radiation beam). Thesensor apparatus may for example be a sensor which measures the currentin the electron beam of the free electron laser FEL. This may forexample be a beam position monitor for which a calibration of outputsignal to electron current has been performed. The current in theelectron beam may be correlated to the EUV radiation beam power (e.g. ifthe conversion efficiency of the free electron laser is constant). Thesensor apparatus may for example be a detector which is used to monitorthe power of a laser beam used to generate electrons for the electronbeam (e.g. as described below in relation to FIG. 4). The power of thelaser beam may be correlated to the EUV radiation beam power. The sensorapparatus may for example be a Faraday cup (or equivalent) configured tomeasure the charge of electrons of the electron beam incident at a beamdump. The charge of electrons incident at the beam dump may for examplebe correlated to the EUV radiation beam power.

The beam delivery system comprises a beam splitting apparatus 19 and mayoptionally also comprise beam expanding optics (not shown). The mainradiation beam B is split into a plurality of radiation beamsB_(a)-B_(n) (which may be referred to as branch beams), each of which isdirected to a different one of the lithographic apparatus LA_(a)-LA_(n),by the beam splitting apparatus 19. The branch radiation beamsB_(a)-B_(n) may be split off from the main radiation beam in series,with each branch radiation beam being split off from the main radiationbeam downstream from the preceding branch radiation beam. Where this isthe case the branch radiation beams may for example propagatesubstantially parallel to each other.

The optional beam expanding optics (not shown) are arranged to increasethe cross sectional area of the radiation beam B. Advantageously, thisdecreases the heat load on mirrors downstream of the beam expandingoptics. This may allow the mirrors downstream of the beam expandingoptics to be of a lower specification, with less cooling, and thereforeless expensive. Additionally or alternatively, it may allow thedownstream mirrors to be nearer to normal incidence. The beam splittingapparatus 19 may comprise a plurality of static extraction mirrors (notshown) arranged in the path of the beam B which direct radiation fromthe main beam B along the plurality of branch radiation beamsB_(a)-B_(n). Increasing the size of the main beam B reduces the accuracywith which the mirrors must be located in the beam B path. Therefore,this allows for more accurate splitting of the output beam B by thesplitting apparatus 19. For example, the beam expanding optics may beoperable to expand the main beam B from approximately 100 μm to morethan 10 cm before the main beam B is split by the beam splittingapparatus 19.

In an embodiment, the branch radiation beams B_(a)-B_(n) are eachdirected through a respective attenuator 15 a-15 n. Each attenuator 15a-15 n is arranged to adjust the intensity of a respective branchradiation beam B_(a)-B_(n) before the branch radiation beam B_(a)-B_(n)passes into its corresponding lithographic apparatus LA_(a)-LA_(n). Eachattenuator 15 a-15 n may be controlled by a controller CTA_(a-n) usingfeedback provided from a lithographic apparatus associated with thatattenuator. For example, a lithographic apparatus LA_(n) may include asensor SL_(n) which monitors the intensity of the branch radiation beamB_(n) within that lithographic apparatus. The output from the sensorSL_(n) may be used to control the attenuator 15 _(n). Thus afeedback-based control loop is provided, as indicated by the dashed lineF2 _(n). The sensor SL_(n) may be provided at any suitable location inthe lithographic apparatus LA_(n). For example, the sensor SL_(n) may belocated after a projection system of the lithographic apparatus (e.g. ata substrate supporting table of the lithographic apparatus).Alternatively, the sensor SL_(n) may be located before a projectionsystem of the lithographic apparatus (e.g. between an illuminationsystem and a mask support structure of the lithographic apparatus).

The first feedback-based control loop F1 may have a faster response thanthe second feedback-based control loop F2 _(n).

The first feedback-based control loop F1 may operate at a frequency of10 kHz or more, e.g. 50 kHz or more. The first feedback-based controlloop may for example operate at a frequency of around 100 kHz or more.The second feedback-based control loop may operate at a frequency of 1kHz or less.

In an embodiment, the controller CT which controls the first feedbackloop may be configured to take into account transmission of for examplethe optics of the beam splitter 19 and/or of the lithographic apparatusLA_(a)-LA_(n), Where this is the case, the controller CT may control thedose of radiation delivered to a lithographic substrate in alithographic apparatus via control of the free electron laser FEL.

In an embodiment, feedback may be provided from a sensor in alithographic apparatus LA_(a)-LA_(n), to the controller CT whichcontrols the free electron laser FEL. The sensor may for example beprovided in an illumination system of the lithographic apparatus.Feedback from sensors in more than one lithographic apparatusLA_(a)-LA_(n), may be provided to the controller CT which controls thefree electron laser FEL.

A target location on a lithographic substrate may receive EUV radiationfor around 1 ms. Controlling the power of EUV radiation delivered to alithographic substrate via feedback-based control of the free electronlaser FEL may provide improved consistency of exposure dose at targetlocations on a lithographic substrate. A feedback-based control loopoperating at a frequency of 10 kHz or more will provide some control ofexposure dose delivered in 1 ms. A feedback-based control loop operatingat a frequency of 50 kHz or more will provide improved control ofexposure dose delivered in 1 ms (it may allow fluctuations in the powerof the EUV radiation beam to be smoothed out more completely). Afeedback-based control loop operating at a frequency of around 100 kHzor more may provide still further improved control of exposure dosedelivered in 1 ms. A feedback-based control loop for the free electronlaser FEL operating at frequencies of 1 MHz or more may not provide anysignificant additional benefit in terms of dose control because the 1 msexposure time is such that EUV radiation fluctuations at suchfrequencies will be averaged out effectively during the exposure time.

The controller CT and/or the controllers CTA_(a-n) may incorporate somefeed-forward control which takes into account changes of radiation beamparameters which are known and which will occur in known circumstances(e.g. radiation beam changes which occur immediately after the freeelectron laser begins operating).

The radiation source SO, beam splitting apparatus 19, beam expandingoptics (if present) and lithographic apparatus LA_(a)-LA_(n) may all beconstructed and arranged such that they can be isolated from theexternal environment. A vacuum may be provided in at least part of theradiation source SO, beam splitting apparatus 19 and lithographicapparatuses LA_(a)-LA_(n) so as to minimise the absorption of EUVradiation. Different parts of the lithographic system LS may be providedwith vacuums at different pressures (i.e. held at different pressureswhich are below atmospheric pressure).

Referring to FIG. 2, a lithographic apparatus LA_(a) comprises anillumination system IL, a support structure MT configured to support apatterning device MA (e.g. a mask), a projection system PS and asubstrate table WT configured to support a substrate W. The illuminationsystem IL is configured to condition the branch radiation beam B_(a)that is received by that lithographic apparatus LA_(a) before it isincident upon the patterning device MA. The projection system PS isconfigured to project the radiation beam B_(a)′ (now patterned by thepatterning device MA) onto the substrate W. The substrate W may includepreviously formed patterns. Where this is the case, the lithographicapparatus aligns the patterned radiation beam B_(a)′ with a patternpreviously formed on the substrate W.

The branch radiation beam B_(a) that is received by the lithographicapparatus LAa passes into the illumination system IL from the beamsplitting apparatus 19 though an opening 8 in an enclosing structure ofthe illumination system IL. Optionally, the branch radiation beam B_(a)may be focused to form an intermediate focus at or near to the opening8.

The illumination system IL may include a facetted field mirror device 10and a facetted pupil mirror device 11. The faceted field mirror device10 and faceted pupil mirror device 11 together provide the radiationbeam B_(a) with a desired cross-sectional shape and a desired angulardistribution. The radiation beam B_(a) passes from the illuminationsystem IL and is incident upon the patterning device MA held by thesupport structure MT. The patterning device MA reflects and patterns theradiation beam to form a patterned beam B_(a)′. The illumination systemIL may include other mirrors or devices in addition to or instead of thefaceted field mirror device 10 and faceted pupil mirror device 11. Theillumination system IL may for example include an array of independentlymoveable mirrors. The independently moveable mirrors may for examplemeasure less than 1 mm across. The independently moveable mirrors mayfor example be microelectromechanical systems (MEMS) devices.

Following redirection (e.g. reflection) from the patterning device MAthe patterned radiation beam B_(a)′ enters the projection system PS. Theprojection system PS comprises a plurality of mirrors 13, 14 which areconfigured to project the radiation beam B_(a)′ onto a substrate W heldby the substrate table WT. The projection system PS may apply areduction factor to the radiation beam, forming an image with featuresthat are smaller than corresponding features on the patterning deviceMA. A reduction factor of 4 may for example be applied. Although theprojection system PS has two mirrors in FIG. 2, the projection systemmay include any number of mirrors (e.g. six mirrors).

The lithographic apparatus LA_(a) is operable to impart a radiation beamB_(a) with a pattern in its cross-section and project the patternedradiation beam onto a target portion of a substrate thereby exposing atarget portion of the substrate to the patterned radiation. Thelithographic apparatus LA_(a) may, for example, be used in a scan mode,wherein the support structure MT and the substrate table WT are scannedsynchronously while a pattern imparted to the radiation beam B_(a)′ isprojected onto a substrate W (i.e. a dynamic exposure). The velocity anddirection of the substrate table WT relative to the support structure MTmay be determined by the demagnification and image reversalcharacteristics of the projection system PS. The patterned radiationbeam B_(a)′ which is incident upon the substrate W may comprise a bandof radiation. The band of radiation may be referred to as an exposureslit. During a scanning exposure, the movement of the substrate table WTand the support structure MT are such that the exposure slit travelsover a target portion of substrate W thereby exposing the target portionof the substrate W to patterned radiation. It will be appreciated that adose of radiation to which a given location within the target portion ofthe substrate W is exposed depends on the power of the radiation beamB_(a)′ and the amount of time for which that location is exposed toradiation as the exposure slit is scanned over the location (the effectof the pattern is neglected in this instance). The term “targetlocation” may be used to denote a location on the substrate which isexposed to radiation (and for which the dose of received radiation maybe calculated).

An attenuator 15 a is shown in FIG. 2, the attenuator being providedbefore the lithographic apparatus. Embodiments of the attenuator 15 aare described further below. The lithographic apparatus may be providedwith a sensor configured to measure the power of the EUV radiation beamin the lithographic apparatus. The sensor may for example be provided inthe illumination system IL, as schematically indicated by dashed lineSL_(a). Additionally or alternatively, the sensor may be provided afterthe projection system. The sensor may for example be provide on thesubstrate table, as indicated schematically by dashed line SL_(a). Acontroller CTA_(a) may control the attenuation provided by theattenuator 15 a. The controller CTA_(a) may receive a signal from thesensor SL_(a) and use this signal, at least in part, to control theattenuation. Thus, a feedback-based control loop may be provided.

Referring again to FIG. 1, the free electron laser FEL is configured togenerate an EUV radiation beam B with sufficient power to supply each ofthe lithographic apparatus LA_(a)-LA_(n). As noted above, the radiationsource may comprise a free electron laser.

A free electron laser comprises an electron source and accelerator,which are operable to produce a bunched relativistic electron beam, anda periodic magnetic field through which the bunches of relativisticelectrons are directed. The periodic magnetic field is produced by anundulator and causes the electrons to follow an oscillating path about acentral axis. As a result of the acceleration caused by the magneticstructure the electrons spontaneously radiate electromagnetic radiationgenerally in the direction of the central axis. The relativisticelectrons interact with radiation within the undulator. Under certainconditions, this interaction causes the electrons to bunch together intomicrobunches, modulated at the wavelength of radiation within theundulator, and coherent emission of radiation along the central axis isstimulated.

The path followed by the electrons may be sinusoidal and planar, withthe electrons periodically traversing the central axis, or may behelical, with the electrons rotating about the central axis. The type ofoscillating path may affect the polarization of radiation emitted by thefree electron laser. For example, a free electron laser which causes theelectrons to propagate along a helical path may emit ellipticallypolarized radiation, which may be desirable for exposure of a substrateW by some lithographic apparatus.

FIG. 3 is a schematic depiction of a free electron laser FEL comprisingan injector 21, a linear accelerator 22, a bunch compressor 23, anundulator 24, an electron decelerator 26 and a beam dump 100.

The injector 21 is arranged to produce a bunched electron beam E andcomprises an electron source such as, for example, a thermionic cathodeor photo-cathode and an accelerating electric field. Electrons in theelectron beam E are further accelerated by the linear accelerator 22. Inan example, the linear accelerator 22 may comprise a plurality of radiofrequency cavities, which are axially spaced along a common axis, andone or more radio frequency power sources, which are operable to controlthe electromagnetic fields along the common axis as bunches of electronspass between them so as to accelerate each bunch of electrons. Thecavities may be superconducting radio frequency cavities.Advantageously, this allows: relatively large electromagnetic fields tobe applied at high duty cycles; larger beam apertures, resulting infewer losses due to wakefields; and for the fraction of radio frequencyenergy that is transmitted to the beam (as opposed to dissipated throughthe cavity walls) to be increased. Or, the cavities may beconventionally conducting (i.e. not superconducting), and may be formedfrom, for example, copper. Other types of linear accelerators may beused. For example, laser wake-field accelerators or inverse freeelectron laser accelerators.

The electron beam E passes through a bunch compressor 23, disposedbetween the linear accelerator 22 and the undulator 24. The bunchcompressor 23 is configured to bunch electrons in the electron beam Eand spatially compress existing bunches of electrons in the electronbeam E. One type of bunch compressor 23 comprises a radiation fielddirected transverse to the electron beam E. An electron in the electronbeam E interacts with the radiation and bunches with other electronsnearby. Another type of bunch compressor 23 comprises a magneticchicane, wherein the length of a path followed by an electron as itpasses through the chicane is dependent upon its energy. This type ofbunch compressor may be used to compress a bunch of electrons which havebeen accelerated in a linear accelerator 22 by a plurality of conductorswhose potentials oscillate at, for example, radio frequencies.

The electron beam E then passes through the undulator 24. Generally, theundulator 24 comprises a plurality of modules. Each module comprises aperiodic magnet structure, which is operable to produce a periodicmagnetic field and is arranged so as to guide the relativistic electronbeam E produced by the injector 21 and linear accelerator 22 along aperiodic path within that module. As a result, within each undulatormodule, the electrons radiate electromagnetic radiation generally in thedirection of a central axis of their periodic path through that module.The undulator 24 may further comprise a mechanism to refocus theelectron beam E, such as a quadrupole magnet in between one or morepairs of adjacent sections. The mechanism to refocus the electron beam Emay reduce the size of the electron bunches, which may improve thecoupling between the electrons and the radiation within the undulator24, increasing the stimulation of emission of radiation.

As electrons move through each undulator module, they interact with theelectric field of the radiation, exchanging energy with the radiation.In general the amount of energy exchanged between the electrons and theradiation will oscillate rapidly unless conditions are close to aresonance condition, given by:

$\begin{matrix}{{\lambda_{em} = {\frac{\lambda_{u}}{2\gamma^{2\;}}\left( {1 + \frac{K^{2}}{A}} \right)}},} & (1)\end{matrix}$where λ_(em) is the wavelength of the radiation, λ_(u) is the undulatorperiod for the undulator module that the electrons are propagatingthrough, γ is the Lorentz factor of the electrons and K is the undulatorparameter. A is dependent upon the geometry of the undulator 24: for ahelical undulator that produces circularly polarized radiation A=1, fora planar undulator A=2, and for a helical undulator which produceselliptically polarized radiation (that is neither circularly polarizednor linearly polarized) 1<A<2. In practice, each bunch of electrons willhave a spread of energies although this spread may be minimized as faras possible (by producing an electron beam E with low emittance). Theundulator parameter K is typically approximately 1 and is given by:

$\begin{matrix}{{K = \frac{q\;\lambda_{u}B_{0}}{2\pi\; m\; c}},} & (2)\end{matrix}$where q and m are, respectively, the electric charge and mass of theelectrons, B₀ is the amplitude of the periodic magnetic field, and c isthe speed of light.

The resonant wavelength γ_(em) is equal to the first harmonic wavelengthspontaneously radiated by electrons moving through each undulatormodule. The free electron laser FEL may operate in self-amplifiedspontaneous emission (SASE) mode. Operation in SASE mode may require alow energy spread of the electron bunches in the electron beam E beforeit enters each undulator module. Alternatively, the free electron laserFEL may comprise a seed radiation source, which may be amplified bystimulated emission within the undulator 24. The free electron laser FELmay operate as a recirculating amplifier free electron laser (RAFEL),wherein a portion of the radiation generated by the free electron laserFEL is used to seed further generation of radiation.

Electrons moving through the undulator 24 may cause the amplitude ofradiation to increase, i.e. the free electron laser FEL may have anon-zero gain. Maximum gain may be achieved when the resonance conditionis met or when conditions are close to but slightly off resonance.

A region around a central axis of each undulator module may beconsidered to be a “good field region”. The good field region may be avolume around the central axis wherein, for a given position along thecentral axis of the undulator module, the magnitude and direction of themagnetic field within the volume are substantially constant. An electronbunch propagating within the good field region may satisfy the resonantcondition of Eq. (1) and will therefore amplify radiation. Further, anelectron beam E propagating within the good field region should notexperience significant unexpected disruption due to uncompensatedmagnetic fields.

Each undulator module may have a range of acceptable initialtrajectories. Electrons entering an undulator module with an initialtrajectory within this range of acceptable initial trajectories maysatisfy the resonant condition of Eq. (1) and interact with radiation inthat undulator module to stimulate emission of coherent radiation. Incontrast, electrons entering an undulator module with other trajectoriesmay not stimulate significant emission of coherent radiation.

For example, generally, for helical undulator modules the electron beamE should be substantially aligned with a central axis of the undulatormodule. A tilt or angle between the electron beam E and the central axisof the undulator module should generally not exceed 1/10ρ, where ρ isthe Pierce parameter. Otherwise the conversion efficiency of theundulator module (i.e. the portion of the energy of the electron beam Ewhich is converted to radiation in that module) may drop below a desiredamount (or may drop almost to zero). In an embodiment, the Pierceparameter of an EUV helical undulator module may be of the order of0.001, indicating that the tilt of the electron beam E with respect tothe central axis of the undulator module should be less than 100 μrad.

For a planar undulator module, a greater range of initial trajectoriesmay be acceptable. Provided the electron beam E remains substantiallyperpendicular to the magnetic field of a planar undulator module andremains within the good field region of the planar undulator module,coherent emission of radiation may be stimulated.

As electrons of the electron beam E move through a drift space betweeneach undulator module, the electrons do not follow a periodic path.Therefore, in this drift space, although the electrons overlap spatiallywith the radiation, they do not exchange any significant energy with theradiation and are therefore effectively decoupled from the radiation.

The bunched electron beam E has a finite emittance and will thereforeincrease in diameter unless refocused. Therefore, the undulator 24further comprises a mechanism for refocusing the electron beam E inbetween one or more pairs of adjacent modules. For example, a quadrupolemagnet may be provided between each pair of adjacent modules. Thequadrupole magnets reduce the size of the electron bunches and keep theelectron beam E within the good field region of the undulator 24. Thisimproves the coupling between the electrons and the radiation within thenext undulator module, increasing the stimulation of emission ofradiation.

An electron which meets the resonance condition as it enters theundulator 24 will lose (or gain) energy as it emits (or absorbs)radiation, so that the resonance condition is no longer satisfied.Therefore, in some embodiments the undulator 24 may be tapered. That is,the amplitude of the periodic magnetic field and/or the undulator periodλ_(u) may vary along the length of the undulator 24 in order to keepbunches of electrons at or close to resonance as they are guided thoughthe undulator 24. The tapering may be achieved by varying the amplitudeof the periodic magnetic field and/or the undulator period λ_(u) withineach undulator module and/or from module to module. Additionally oralternatively tapering may be achieved by varying the helicity of theundulator 24 (thereby varying the parameter A) within each undulatormodule and/or from module to module.

After leaving the undulator 24, the electron beam E is absorbed by adump 100. The dump 100 may comprise a sufficient quantity of material toabsorb the electron beam E. The material may have a threshold energy forinduction of radioactivity. Electrons entering the dump 100 with anenergy below the threshold energy may produce only gamma ray showers butwill not induce any significant level of radioactivity. The material mayhave a high threshold energy for induction of radioactivity by electronimpact. For example, the beam dump may comprise aluminium (Al), whichhas a threshold energy of around 17 MeV. It is desirable to reduce theenergy of electrons in the electron beam E before they enter the dump100. This removes, or at least reduces, the need to remove and disposeof radioactive waste from the dump 100. This is advantageous since theremoval of radioactive waste requires the free electron laser FEL to beshut down periodically and the disposal of radioactive waste can becostly and can have serious environmental implications.

The energy of electrons in the electron beam E may be reduced beforethey enter the dump 100 by directing the electron beam E through adecelerator 26 disposed between the undulator 24 and the beam dump 100.

In an embodiment the electron beam E which exits the undulator 24 may bedecelerated by passing the electrons back through the linear accelerator22 with a phase difference of 180 degrees relative to radio frequency(RF) fields in the linear accelerator 22. The RF fields in the linearaccelerator therefore serve to decelerate the electrons which are outputfrom the undulator 24. As the electrons decelerate in the linearaccelerator 22 some of their energy is transferred to the RF fields inthe linear accelerator 22. Energy from the decelerating electrons istherefore recovered by the linear accelerator 22 and may be used toaccelerate the electron beam E output from the injector 21. Such anarrangement is known as an energy recovering linear accelerator (ERL).

FIG. 4 is a schematic depiction of an embodiment of the injector 21. Theinjector 21 comprises an electron gun 31 and an electron booster 33. Theelectron gun 31 is arranged to support a photocathode 43 inside a vacuumchamber 32. The electron gun 31 is further arranged to receive a beam ofradiation 41 from a radiation source 35. The radiation source 35 may,for example, comprise a laser 35 which emits a laser beam 41. The laserbeam 41 is directed into the vacuum chamber 32 through a window 37 andis incident on the photocathode 43. In the embodiment shown in FIG. 4,the laser beam 41 is reflected by a mirror 39 such that it is incidenton the photocathode 43.

The photocathode 43 is held at a high voltage. For example, thephotocathode 43 may be held at a voltage of approximately severalhundred kilovolts. The photocathode 43 may be held at a high voltage byusing a voltage source which may form part of the electron gun 32 or maybe separate from the electron gun 32. Photons in the laser beam 41 areabsorbed by the photocathode 43 and excite electrons in the photocathode43. Some electrons in the photocathode 43 are excited to a high enoughenergy state that they are emitted from the photocathode 43. The highvoltage of the photocathode 43 is negative and thus serves to accelerateelectrons which are emitted from the photocathode 43 away from thephotocathode 43, thereby forming a beam of electrons E.

The laser beam 41 is a pulsed laser beam. Electrons are emitted from thephotocathode 43 in bunches which correspond to the pulses of the laserbeam 41. The electron beam E therefore comprises a series of electronbunches 42. The laser 35 may, for example, be a picosecond laser andthus pulses in the laser beam 41 may have a duration of approximately afew picoseconds. The voltage of the photocathode 43 may be a DC voltageor an AC voltage. In embodiments in which the voltage of thephotocathode 43 is an AC voltage the frequency and phase of thephotocathode voltage may be matched with pulses of the laser beam 41such that pulses of the laser beam 41 coincide with peaks in the voltageof the photocathode 43. Pulses of the laser beam 41 may be matched withaccelerating fields in the electron booster 33 and the linearaccelerator 22 such that electron bunches 42 arrive at the electronbooster 33 and the linear accelerator 22 at a time at which theaccelerating fields act to accelerate the electron bunches 42.

The electron beam E which is emitted from the photocathode 43 isaccelerated by the electron booster 33. The electron booster 33 servesto accelerate the electron bunches along a beam passage 34 and towardsthe linear accelerator 22 (not shown in FIG. 4) which furtheraccelerates the electron bunches to relativistic speeds (as wasdescribed above). The electron booster 33 may, for example, accelerateelectron bunches 42 to energies in excess of approximately 5 MeV. Insome embodiments the electron booster 33 may accelerate electron bunches42 to energies in excess of approximately 10 MeV. In some embodimentsthe electron booster 33 may accelerate electron bunches 42 to energiesof up to approximately 20 MeV.

The electron booster 33 may be similar to the linear accelerator 22 andmay, for example, comprise a plurality of radio frequency cavities 47(depicted in FIG. 4) and one or more radio frequency power sources (notshown). The radio frequency power sources may be operable to controlelectromagnetic fields in the beam passage 34. As bunches of electrons42 pass between the cavities 47, the electromagnetic fields controlledby the radio frequency power sources cause each bunch of electrons toaccelerate. The cavities 47 may be superconducting radio frequencycavities. Alternatively, the cavities 47 may be conventionallyconducting (i.e. not superconducting), and may be formed from, forexample, copper.

As was described above each pulse of the laser beam 41 which is incidenton the photocathode 43 causes a corresponding electron bunch 42 to beemitted from the photocathode 43. Each electron bunch 42 in the electronbeam E is accelerated by the electron booster 33 and by the linearaccelerator 22. The accelerated electron bunches 42 pass into theundulator 24 where they stimulate emission of radiation to form aradiation beam B. The radiation beam B is a pulsed radiation beam witheach electron bunch 42 in the undulator 24 causing emission of a pulseof radiation in the radiation beam B. For each pulse in the laser beam41 there is therefore a corresponding electron bunch 42 in the electronbeam E and a corresponding pulse in the radiation beam B which isemitted from the free electron laser FEL.

The free electron laser FEL may form part of the lithographic system LSof FIG. 1, wherein radiation produced by the free electron laser isultimately received by one or more substrates W within one or morelithographic apparatus LAa-LAn. These substrates W may be considered tocomprise target portions which are arranged to receive patternedradiation. Within the lithographic system LS, radiation is transportedfrom the free electron laser FEL to the substrates via: (i) a beamdelivery system (for example comprising beam expanding optics 20 and thebeam splitting apparatus 19); and (ii) optics within the lithographicapparatuses LAa-LAn (for example optics 10, 11, 13, 14). The beamdelivery system and the optics within a lithographic apparatus may bereferred to as an optical path which is configured to transportradiation from the free electron laser FEL to a substrate W. The opticalpath reflects and/or transmits radiation so as to provide a dose ofradiation at the substrate W. The fraction of the radiation beam B whichpropagates through the optical path and which is incident on a substrateW may be referred to as the transmittance of the optical path. It willbe appreciated that an optical path may include reflective and/ortransmissive elements and the transmittance of the optical path dependson the reflectivity of any reflective elements in the optical path aswell as the transmittance of any transmissive elements in the opticalpath. The transmittance of the optical path may additionally depend on amatching of the cross-section of the radiation beam B with opticalelements on which the radiation beam is incident in the optical path.For example, an optical element (e.g. a mirror) in the optical path mayhave a smaller cross-section than the cross-section of the radiationbeam B which is incident of the optical element. Portions of thecross-section of the radiation beam B which lie outside of thecross-section of the optical element may therefore be lost from theradiation beam (e.g. by not being reflected by a mirror) and maytherefore reduce the transmittance of the optical path.

It may desirable to control the dose of radiation which is received bytarget locations on substrates W in the lithographic apparatusLA_(a)-LA_(n) of the lithographic system LS. In particular it may bedesirable to control the dose of radiation such that each targetlocation of a given target portion on a substrate receives substantiallythe same dose of radiation.

As was described above with reference to FIG. 2 a dose of radiationwhich is received by a target location of a substrate W depends on thepower of the radiation beam to which the target location is exposed(e.g. patterned radiation beam B_(a)′) and the amount of time for whichthe target location of the substrate W is exposed to the radiation beam.The power of a patterned radiation beam B_(a)′ in a lithographicapparatus LA_(a) depends on the power of the radiation beam B which isemitted by the free electron laser FEL and the transmittance of theoptical path between the free electron laser FEL and the substrate W. Adose of radiation which is received at a target location of a substratemay therefore be controlled by controlling the power of the radiationbeam B which is emitted from the free electron laser FEL and/or bycontrolling the transmittance of the optical path between the freeelectron laser FEL and the substrate W. The power of the radiation beamB emitted from the free electron laser FEL may be controlled using afeedback-based control loop (as described above in relation to FIG. 1).

The power of the radiation beam B which is emitted from the freeelectron laser FEL may, for example, be controlled by controlling one ormore properties of the free electron laser FEL (e.g. using feedback froma sensor which measures radiation beam power). For example, theconversion efficiency (with which power in the electron beam E isconverted to power in the radiation beam B) of the undulator, the energyof the electron beam E and/or another property of the free electronlaser FEL may be controlled. However many of the properties of a freeelectron laser FEL and the radiation beam B which is emitted by the freeelectron laser FEL may be interlinked and thus changing one property maycause an undesirable change in another property. For example, a changein the conversion efficiency of the undulator and/or in the energy ofthe electron beam E may result in changes in the wavelength, thebandwidth and/or the spatial intensity distribution of the radiationbeam B. The transmittance of the optical path between the free electronlaser FEL and a substrate W may be strongly dependent on properties ofthe radiation beam B such as the wavelength, bandwidth and/or spatialintensity distribution of the radiation beam B. Changes in properties ofthe radiation beam B (e.g. wavelength, bandwidth, spatial intensitydistribution) may therefore disadvantageously result in unwanted changesin the dose of radiation which is received at a substrate W.

The dose of radiation which is received at a target location on thesubstrate may alternatively be controlled without affecting otherproperties of the radiation beam B (e.g. wavelength, bandwidth, spatialintensity distribution) by controlling the amount of time for which thethat location on the substrate W is exposed to radiation

In an embodiment, the lithographic apparatus may be configured such thata target portion of a substrate W is exposed by scanning the substraterelative to band of radiation which extends across the target portiontransverse to the scanning direction. The band of radiation may bereferred to as an exposure slit. The dose of radiation received at atarget location on the substrate W depends on an exposure time periodduring which a radiation beam (e.g. patterned radiation beam B_(a)′) isdirected onto that target location, and the number and duration ofpulses which occur in the radiation beam during the exposure timeperiod. For example, in a scanning lithographic apparatus, the amount oftime for which a target location of the substrate W is exposed to aradiation beam depends on the time taken for the exposure slit to travelover that location. The dose of radiation which is received at thetarget location depends upon the number of pulses of the radiation beamwhich occur during that exposure time period and the average energywhich is delivered to the target location with each pulse. In anembodiment, a wafer may be scanned relative to the exposure slit suchthat the exposure time period is approximately 1 ms. In otherembodiments the exposure time period may be greater than 1 ms and may,for example, be as long as 5 ms (e.g. due to slower scanning movement ofthe wafer relative to the exposure slit).

In some embodiments the dose of radiation which is received at a targetlocation on a substrate W may be controlled by controlling the number ofpulses of radiation which are incident on the target location during anexposure time period of that location. Since a radiation beam (e.g.patterned radiation beam B_(a)′) which is incident on a substrate Woriginates from the radiation beam B which is emitted from the freeelectron laser FEL, the number of pulses of the radiation beam which isincident on the substrate W during an exposure time period depends onthe number pulses of the radiation beam B during the exposure timeperiod. As was described above, the pulses in the radiation beam Bcorrespond to pulses of the laser beam 41 which are incident on thephotocathode 43 and correspond to electron bunches 42 which are emittedfrom the photocathode 43 and which stimulate emission of radiation inthe undulator 24. The number of pulses of radiation which are incidenton a target location of a substrate W during an exposure time period maytherefore be controlled by controlling the number of pulses in the laserbeam 41 which is incident on the photocathode 43 and/or the number ofelectron bunches 42 which propagate through the undulator 24, during theexposure time period.

Controlling the number of pulses of radiation which are incident upon atarget location of a substrate W may be considered to be equivalent tocontrolling the power of radiation incident upon the target location ofthe substrate. This may, at least in part, be achieved by controllingthe power of the radiation beam B output from the free electron laser.The power of the radiation beam output from the free electron laser maybe controlled by controlling the number of pulses in the radiation beamoutput from the free electron laser.

A first control apparatus 51, which is depicted in FIG. 4, may be usedto control the number of pulses of the laser beam 41 which are incidenton the photocathode 43 during an exposure time period and thereforecontrol the number of electron bunches 42 which form the electron beam Eduring the exposure time period. Also depicted in FIG. 4 is a secondcontrol apparatus 52 which may be used to control the number of electronbunches 42 which are output from the injector 21 during an exposure timeperiod. The first and/or second control apparatus 51, 52 may usefeedback from a sensor apparatus ST (see FIG. 1) which measures thepower of the EUV radiation beam output from the free electron laser(thus providing a feedback-based control loop). This controls the powerof the radiation beam output from the free electron laser.

The first control apparatus 51 and/or the second control apparatus 52may be used to control the number of electron bunches 42 which propagatethrough the undulator 24 during an exposure time period and thereforethe number of pulse of the radiation beam B which are stimulated duringthe exposure time period and the number of pulses of radiation which arereceived at a substrate W during the exposure time period. The firstcontrol apparatus 51 and/or the second control apparatus 52 maytherefore be used to control a dose of radiation which is received by atarget location of a substrate W during an exposure time period.Advantageously, controlling the dose of radiation by controlling thenumber of pulses of the radiation beam B which occur during an exposuretime period has little or no effect on other properties of the radiationbeam B such as the wavelength, bandwidth or spatial intensitydistribution of the radiation beam B. Controlling the dose of radiationby controlling the number of pulses of the radiation beam B which occurduring an exposure time period is additionally advantageous because itallows the dose to be adjusted quickly, for example, as part of afeedback-based control loop which responds to one or more measurementsof the radiation beam power. The feedback-based control loop may forexample operate at a frequency of 10 kHz or more.

In some embodiments a dose of radiation which is received by a targetlocation of a substrate W may be controlled by substantially preventingone or more pulses in the laser beam 41 from being incident on thephotocathode 43. The one or more pulses in the laser beam 41 may besubstantially prevented from being incident on the photocathode 43 bythe first control apparatus 51.

FIG. 5 is a schematic depiction of an embodiment of the first controlapparatus 51 which comprises a Pockels cell 61, a voltage source 63 anda polarizer 65. The first control apparatus 51 receives the laser beam41 from the laser 35 such that the laser beam 41 is incident on thePockels cell 61. The Pockels cell 61 comprises an electro-optic crystal62 and a pair of electrodes 64. The electrodes 64 are electricallycoupled to the voltage source 63 by wires 67 such that the voltagesource 63 is operable to generate a potential difference between theelectrodes 64 and create an applied electric field in the electro-opticcrystal 62. The refractive index of the electro-optic crystal 62 ismodified in proportion to the applied electric field such that thepotential difference between the electrodes 64 may be controlled inorder to bring about a desired rotation of the polarization state of thelaser beam 41 as it propagates through the electro-optic crystal 62.

The polarizer 65 is configured to only transmit radiation having a givenpolarization state. The voltage source 63 may be operated so as tocontrol the polarization state of the laser beam 41 which is incident onthe polarizer 65 and therefore control the amount of radiation from thelaser beam 41 which is transmitted though the polarizer 65. The laserbeam 41 which is incident on the Pockels cell 61 is linearly polarized.The polarizer 65 may, for example, be configured to only transmitradiation having the polarization state of the laser beam 41 before thelaser beam 41 is incident on the Pockels cell 61. During normaloperation the voltage source 63 may not generate a potential differencebetween the electrodes 64 such that the polarization state of the laserbeam 41 is unchanged as it propagates through the electro-optic crystal62 and thus substantially all of the laser beam 41 is transmitted by thepolarizer 65. Radiation which is transmitted by the polarizer 65propagates out of the first control apparatus 51 and is directed to beincident on the photocathode 43 (as is shown in FIG. 4).

At times it may be desirable to prevent a pulse of the laser beam 41from being incident on the photocathode 43 or to reduce the energy of apulse of the laser beam 41 which is incident on the photocathode 43. Forexample, it may be desirable to reduce the energy of a pulse of thelaser beam 41 which is incident on the photocathode 43 such that thepulse has an energy of approximately 10% or less of a regular pulse ofthe laser beam 41 which is incident on the photocathode 43. The voltagesource 63 is operable to generate a potential difference between theelectrodes 64 which causes the polarization state of the laser beam 41to be rotated by around 90° as it propagates through the electro-opticcrystal 62. The laser beam 41 may therefore be substantially blocked bythe polarizer 65 and thus substantially no radiation from the laser beam41 is incident on the photocathode 43.

The voltage source 63 is operable to switch the Pockels cell 61 betweena first mode of operation during which no potential difference isgenerated between the electrodes 64 and a second mode of operationduring which a potential difference is generated between the electrodes64. During the first mode of operation pulses of the laser beam 41 aretransmitted by the polarizer 65, are incident on the photocathode 43 andcause emission of electron bunches 42 from the photocathode 43. Duringthe second mode of operation the polarization state of pulses of thelaser beam 41 is rotated such that they are not transmitted by thepolarizer 65. Thus, the pulses of the laser beam 41 are not incident onthe photocathode 43 and no corresponding electron bunches 42 are emittedfrom the photocathode 43. Preventing one or more pulses of the laserbeam 41 from being incident on the photocathode 43 by switching thePockels cell 61 to the second mode of operation means that no electronbunches 42 corresponding to the blocked pulses of the laser beam 41 areemitted from the photocathode 43. The electron beam E is thereforeinterrupted so that one or more pulses of the laser beam 41 have noassociated electron bunch 42 in the electron beam E. An interruption ofthe electron beam E causes an interruption of the electron bunches 42which propagate through the undulator 24 and thus causes an interruptionof the stimulation of emission of radiation pulses in the undulator 24.The pulses of the radiation beam B which are emitted from the freeelectron laser FEL are therefore interrupted.

It will be appreciated that interrupting the pulses of the radiationbeam B which are emitted from the free electron laser FEL will cause aninterruption of pulses of radiation which are incident on a targetlocation of a substrate W, thereby reducing the dose of radiation whichis incident at the target location during an exposure time period of thetarget location (e.g. the time during which an exposure slit is scannedover that location). The voltage source 63 of the first controlapparatus 51 may be controlled in order to control the number of pulsesof radiation which are incident on a target location of a substrate Wduring an exposure time period in order to control the dose of radiationwhich is received by that location. The voltage source 63 may, forexample, be controlled by a controller CT (which may correspond with thecontroller CT shown in FIG. 1).

In an ideal application the laser beam 41 is perfectly linearlypolarized and the polarizer 65 is configured to transmit radiationhaving the polarization state of the laser beam 41 which is incident onthe Pockels cell 61. In such an ideal application when the Pockels cell61 is in the second mode of operation the polarization state of thelaser beam 41 is rotated by 90° and no radiation from the laser beam 41will be transmitted by the polarizer 65 and no pulse of the laser beam41 will be incident on the photocathode 43. However, in practice thelaser beam 41 may be slightly depolarized and may include a smallcomponent which has a perpendicular polarization state to thepolarization state which is transmitted by the polarizer 65. When thePockels cell 61 is in the second mode of operation the polarizationstate of the perpendicularly polarized component may be rotated by 90°such that it is transmitted by the polarizer 65. A small amount ofradiation from the laser beam 41 may therefore be transmitted by thepolarizer 65 and may be incident on the photocathode 43 even when thePockels cell 61 is in the second mode of operation. For example, a lowpower pulse of the laser beam 41 may be incident on the photocathode 43when the Pockels cell 61 is in the second mode of operation. The lowpower pulse may have a power which is approximately 10% or less of thepower of a pulse of the laser beam 41 which is incident on thephotocathode 43 when the Pockels cell is in the first mode of operation.

A low power pulse of the laser beam 41 which is incident on thephotocathode 43 may cause a low charge electron bunch to be emitted fromthe photocathode 43. The low charge electron bunch may have a peakcurrent which is approximately 10% or less of the peak current of anominal electron bunch 42 which is emitted from the photocathode 43 whenthe Pockels cell is in the first mode of operation. A nominal electronbunch emitted in the first mode of operation may be referred to as atypical electron bunch 42.

In the undulator 24 the power of a pulse of the radiation beam B whichis stimulated by a given electron bunch 42 is a function of the chargeof the electron bunch 42 and the gain of radiation which is caused bythe electron bunch 42 in the undulator 24. The gain which is caused byan electron bunch 42 in the undulator 24 is represented by a so-calledgain length of the electron bunch 42. The gain length of an electronbunch 42 is representative of the length of an undulator section throughwhich the electron bunch 42 must propagate in order to cause a givengain of radiation in the undulator 24. A typical electron bunch 42 whichis emitted from the photocathode 43 when the Pockels cell 61 is in thefirst mode of operation may propagate for approximately 15-25 gainlengths in the undulator 24. The gain length of an electron bunch isproportional to one over the cubed root of the peak current of theelectron bunch. A low charge electron bunch which has a peak current ofapproximately 10% of the peak current of a typical electron bunch 42will therefore have a gain length which is approximately 2-3 timeslarger than the gain length of the typical electron bunch 42. For anundulator 24 of a given length the gain of a low current electron bunchwill therefore be approximately 2-3 times less than the gain of atypical electron bunch 42.

A low charge electron bunch which is emitted from the photocathode 43when the Pockels cell 61 is in the second mode of operation may have acharge which is approximately 10% of the charge of a typical electronbunch 42 and may cause a gain of radiation in the undulator 24 which isapproximately 2-3 times less than the gain caused by a typical electronbunch. A pulse in the radiation beam B which is stimulated by a lowcurrent electron bunch may therefore have an energy which is less thanabout 0.1% of the energy of a pulse in the radiation beam B which isstimulated a typical electron bunch 42. Additionally, a pulse in theradiation beam B which is stimulated by a low current electron bunch mayhave a larger divergence (e.g. a divergence which is approximately 10times larger) than the divergence of a pulse in the radiation beam Bwhich is stimulated by a typical electron bunch 42. The combination ofthese factors means that a dose of radiation which is received at atarget portion of a substrate W due to a pulse in the radiation beam Bwhich is stimulated by a low current electron bunch is negligible whencompared to the dose of radiation which is received due to a pulse inthe radiation beam B which is stimulated by a typical current electronbunch 42. A low current electron bunch in the electron beam E maytherefore be considered to have a negligible effect in the free electronlaser FEL. The first control apparatus 51 may therefore be considered tointerrupt the electron beam E so as to cause at least one pulse of thelaser beam 41 to have substantially no associated electron bunch 42 inthe electron beam E which is output from the injector 21 andsubstantially no associated pulse in the radiation beam B.

In some embodiments the power of a low power pulse of the laser beam 41and the peak current of a low charge electron bunch may be reduced bypositioning a second polarizer in the path of the laser beam 41 beforethe laser beam is incident on the Pockels cell 61. The second polarizermay be configured to only transmit radiation having the samepolarization state as the radiation which is transmitted by thepolarizer 65 and may act to reduce any depolarization of the laser beam41 which is incident on the Pockels cell 61.

Whilst the polarizer 65 is configured to only transmit radiation havinga given polarization state the polarizer 65 may also transmit someradiation with other polarization states. For example, approximately1-0.1% of radiation with other polarization states may be transmitted bythe polarizer 65. This radiation may be referred to as radiation whichis leaked by the polarizer 65. When the Pockels cell 61 is in the secondmode of operation such that the majority of the laser beam 41 is blockedby the polarizer 65 some radiation from the laser beam 41 may thereforestill be leaked by the polarizer 65 such that some radiation from thelaser beam 41 is still incident on the photocathode 43.

In some embodiments a frequency doubling crystal may be positioned inthe path of the laser beam 41 which acts to double the frequency (andhalve the wavelength) of the laser beam 41. For example, the laser 35may be an Nd:YAG laser which emits a laser beam 41 having a wavelengthwhich is approximately 1064 nm. After passing through the frequencydoubling crystal the laser beam 41 may therefore have a wavelength whichis approximately 532 nm.

Typically a frequency doubling crystal only acts to double the frequencyof radiation which has a given polarization state. The frequencydoubling crystal may, for example, be positioned in the optical path ofthe laser beam 41 after the laser beam 41 has been transmitted by thepolarizer 65. The frequency doubling crystal may be configured to doublethe frequency of radiation which has the polarization state which thepolarizer 65 is configured to transmit. In the event that the polarizer65 transmits some radiation having a different polarization state (e.g.when the Pockels cell 61 is in the second mode of operation) thefrequency doubling crystal will not therefore double the frequency ofthis leaked radiation since it does not have the polarization stateneeded for frequency doubling to occur. The frequency doubling crystalmay therefore act as an additional polarizer and may prevent the leakedradiation from being incident on the photocathode 43.

Additionally, a frequency doubling crystal may act to increase thecontrast between the power of a low power pulse of the laser beam 41which is transmitted by the polarizer 65 when the Pockels cell 61 is inthe second mode of operation, and a higher power pulse of the laser beam41 which is transmitted by the polarizer 65 when the Pockels cell 61 isin the first mode of operation. The conversion efficiency with which afrequency doubling crystal converts a pulse of the laser beam 41 to apulse having double the frequency is proportional to the power of thepulse of the laser beam 41. A low power pulse will therefore beconverted with a lower conversion efficiency than a higher power pulseand thus the contrast between a low power pulse and a higher power pulsewill be increased by the frequency doubling crystal.

Advantageously, using a Pockels cell 61 allows fast switching betweenblocking pulses of the laser beam 41 and allowing pulses of the laserbeam 41 to be incident on the photocathode 43. It may be desirable forthe voltage source 63 to be operable to switch the Pockels cell 61between the first mode of operation and the second mode of operationfast enough such that only a single pulse of the laser beam 41 isprevented from being incident on the photocathode 43. For example, itmay be desirable for the voltage source to switch the Pockels cell 61 tothe second mode of operation for a time period in which one pulse of thelaser beam 41 propagates through the Pockels cell 61 such that the pulseof the laser beam 41 is blocked by the polarizer 65 and then switch thePockels cell 61 to the first mode of operation before a subsequent pulseof the laser beam 41 propagates through the Pockels cell 61 such thatthe subsequent pulse of the laser beam 41 is transmitted by thepolarizer 65. Such fast switching of the Pockels cell 61 allows forcontrol over each individual pulse of the laser beam 41.

The pulse repetition frequency of the laser beam 41 may be greater than100 MHz. For example, the pulse repetition frequency of the laser beam41 may be approximately 300 MHz. The spacing between consecutive pulsesof the laser beam 41 as they propagate through the Pockels cell 61 maybe approximately 70 cm. The length of the electro-optic crystal 62 maybe approximately 100 mm and thus only a single pulse of the laser beam41 propagates through the electro-optic crystal 62 at any one time. Thisallows the Pockels cell 61 to be switched between different modes ofoperation for each pulse of the laser beam 41.

The potential difference which is generated between the electrodes 64 inthe second mode of operation (in order to bring about a rotation of thepolarization state of the laser beam 41 of around 90°) may beapproximately 100 V. The amount of time which it takes to generate adesired potential difference (e.g. 100 V) between the electrodes 64 isproportional to the capacitance of the Pockels cell 61 and the impedanceof the cables 67 which connect voltage source 63 to the electrodes 64.The capacitance of the Pockels cell 61 is a function of the surface areaof the electrodes, the separation between the electrodes and of therelative permittivity of the electro-optic crystal 62.

In an embodiment the relative permittivity of the electro-optic crystal62 may lie in an approximate range of about 20-50. The surface area ofeach electrode 64 may be approximately 500 mm² and the separationbetween the electrodes 64 may be approximately 5 mm. The correspondingcapacitance of the Pockels cell may be approximately 50 pF. Theimpedance of the cables 67 may be approximately 50 Ohms. In such anembodiment the amount of time which it takes to generate a desiredpotential difference of 100V between the electrodes 64 may beapproximately 2.5 nanoseconds. Such a time period may be comparable tothe delay between consecutive pulses of the laser beam 41 arriving atthe Pockels cell 61 and thus such a time period may be too long to beable to switch the Pockels cell between the first and second modes ofoperation between consecutive pulses.

The amount of time which it takes to generate a desired potentialdifference (e.g. 100V) between electrodes 64 may be reduced by providingthe Pockels cell with a plurality of pairs of electrodes 64 eachconnected to an independent voltage source 63. For example, in someembodiments a Pockels cell may be provided with 5 or more pairs ofelectrodes 64. In some embodiments a Pockels cell may be provided withup to about 10 pairs of electrodes 64. Each of the plurality of pairs ofelectrodes 64 may have a reduced surface area and thus the capacitancebetween each of the plurality of electrodes 64 may be reduced. In anembodiment in which a given surface area is split evenly amongst aplurality of pairs of electrodes 64 then the capacitance between eachpair of electrodes 64 is reduced by a factor which is equal to thenumber of pairs of electrodes 64, when compared to an embodiment inwhich a single pair of electrodes 64 covers the given surface area. Forexample, in an embodiment in which 5 pairs of electrodes are provided tocover a given surface area the capacitance between each pair ofelectrodes is approximately 5 times less than the capacitance between apair of electrodes 64 in an embodiment in which a single pair ofelectrodes covers the same surface area.

A reduction in the capacitance between each pair of electrodes 64reduces the amount of time which it takes to generate a desiredpotential difference between each pair of electrodes 64. Each pair ofelectrodes 64 may be provided with an independent voltage source 63. Theindependent voltage sources 63 may operate synchronously to each othersuch that the potential difference between each pair of electrodes 64may be substantially the same as each other. A plurality of voltagesources 63 may, for example, be synchronized to within less than about 1picosecond of each other.

A Pockels cell 61 comprising a plurality of pairs of electrodes 64 maybe switched between a first mode of operation in which no potentialdifference is generated between the pairs of electrodes 64 and a secondmode of operation in which a desired potential difference issynchronously generated between each of the pairs of electrodes 64.Providing a Pockels cell 61 with a plurality of pairs of electrodes 64such that the capacitance between each pair of electrodes 64 is reduced,reduces the amount of time which it takes to switch the Pockels cell 61between the first mode of operation and the second mode of operation.For example, a Pockels cell 61 comprising a plurality of pairs ofelectrodes 64 may be switched between the first mode of operation andthe second mode of operation in a time period which is less thanapproximately 1 nanosecond. This may allow the Pockels cell to beswitched between the first and second modes of operation between thearrival of subsequent pulses of the laser beam 41 at the Pockels cell 61and may therefore allow the mode of operation of the Pockels cell 61 tobe switched for each individual pulse of the laser beam 41.

In alternative embodiments the first control apparatus 51 may comprise aplurality of Pockels cells 61 through which the laser beam 41 propagatesbefore being incident on the polarizer 65. Each of the plurality ofPockels cells 61 may, for example, have a reduced length when comparedto an embodiment comprising a single Pockels cell 61. For example, theplurality of Pockels cells 61 may each have a length of approximately 10mm. Each of the plurality of Pockels cells 61 may comprise one or morepairs of electrodes 64 each coupled to an independent voltage source 63.The voltage sources 63 are operable to switch the Pockels cells 61between a first mode of operation in which no potential difference isgenerated between pairs of electrodes 64 and a second mode of operationin which a potential difference is generated between each pair ofelectrodes. The potential difference between each pair of electrodes 64when the Pockels cells 61 are in the second mode of operation may besuch that each Pockels cell rotates the polarization state of the laserbeam 41 by an angle which is less than 90°. The plurality of Pockelscells 61 may be configured such that when each of the plurality ofPockels cells 61 is in the second mode of operation and the laser beam41 propagates through each of the Pockels cells 61, the combined effectof the plurality of Pockels cells 61 is to rotate the polarization stateof the laser beam by around 90° such that substantially none of thelaser beam 41 is transmitted by the polarizer 65.

The potential difference which is generated between pairs of electrodes64 in a Pockels cell 61 which is configured to rotate the polarizationstate of the laser beam 41 by less than 90° when in a second mode ofoperation may be less than a potential difference between a pair ofelectrodes 64 which is configured to rotate the polarization state ofthe laser beam 41 by 90° (e.g. the Pockels cell depicted in FIG. 5). Inan embodiment in which a plurality of Pockels cells 61 which each rotatethe polarization state of the laser beam 41 by less than 90°, smallerpotential differences are therefore generated between pairs ofelectrodes 64 when switching between a first mode of operation and asecond mode of operation than in an embodiment in which a single Pockelscell 61 rotates the polarization state of the laser beam 41 by 90°. Thismay advantageously reduce the amount of time which it takes to switch aPockels cell 61 between a first mode of operation and a second mode ofoperation. As was described above a reduction of the amount of timewhich it takes to switch a Pockels cell 61 between a first mode ofoperation and a second mode of operation may allow Pockels cells 61 tobe switched between the first and second modes of operation between thearrival of subsequent pulses of the laser beam 41 at the Pockels cell 61and may therefore allow the mode of operation of the Pockels cell 61 tobe switched for each individual pulse of the laser beam 41.

In an alternative but equivalent embodiment of the first controlapparatus 51, the polarizer 65 may be configured to only transmitradiation having a polarization state which is orthogonal to thepolarization state of the laser beam 41 before the laser beam 41 isincident on the Pockels cell 61. In such an embodiment, the laser beam41 will be blocked by the polarizer 65 during times at which the voltagesource 63 does not apply a voltage between the electrodes 64. Duringtimes at which the voltage source 63 does apply a voltage between theelectrodes 64, the Pockels cell 61 rotates the polarization state of thelaser beam 41 such that the laser beam 41 is transmitted by thepolarizer 65 and is incident on the photocathode 43.

FIG. 6 is a schematic depiction of an alternative embodiment of a firstcontrol apparatus 51′. The first control apparatus 51′ which is depictedin FIG. 6 comprises a first polarizer 65 a, a second polarizer 65 b, afirst Pockels cell 61 a and a second Pockels cell 61 b. The first andsecond Pockels cells 61 a, 61 b each comprise an electro-optic crystal62 a, 62 b and a pair of electrodes 64 a, 64 b. The pairs of electrodes64 a, 64 b are electrically coupled to voltage sources 63 a, 63 b bywires 67 a, 67 b as is shown in FIG. 6.

The first polarizer 65 a is configured to transmit radiation having thepolarization state of the laser beam 41 which is incident on the firstpolarizer 65 a. The first polarizer 65 a may serve to reduce anydepolarization of the laser beam 41. The first and second Pockels cells61 a, 61 b are each operable to switch between a first mode of operationin which no potential difference is generated between the respectivepair of electrodes 64 a, 64 b and a second mode of operation in which apotential difference is generated between the respective pair ofelectrodes 64 a, 64 b such that the polarization state of the laser beam41 is rotated by around 90° whilst propagating through the respectivePockels cell 61 a, 61 b. The first and second Pockels cells 61 a, 61 bmay be independently switched between the first and second modes ofoperation by the independent voltage sources 63 a, 63 b.

The second polarizer 65 b is configured to only transmit radiation whichhas a polarization state which is orthogonal to the polarization stateof radiation which is transmitted by the first polarizer 65 a. If boththe first Pockels cell 61 a and the second Pockels cell 61 b are in thefirst mode of operation then the polarization state of the laser beam 41will not be rotated by either of the Pockels cells 61 a, 61 b and thelaser beam 41 will not be transmitted by the second polarizer 65 b. Ifone (but not both) of the first or second Pockels cells 61 a, 61 b is inthe second mode of operation then the polarization state of the laserbeam 41 is rotated by around 90° between the first and second polarizers65 a, 65 b and the laser beam 41 is transmitted by the second polarizer65 b. If both of the first Pockels cell 61 a and the second Pockels cell61 b are in the second mode of operation then the polarization state ofthe laser beam 41 is rotated by 180° between the first and secondpolarizers 65 a, 65 b and the laser beam 41 is not transmitted by thesecond polarizer 65. Therefore in order for the laser beam 41 to betransmitted by the second polarizer 65, one (but not both) of the firstor second Pockels cells 61 a, 61 b must be in the second mode ofoperation. The voltage sources 63 a, 63 b may be controlled by acontroller CT (which may correspond with the controller CT shown inFIG. 1) in order to control how many pulses of the laser beam 41 aretransmitted by the second polarizer 65 b such that they are incident onthe photocathode 43.

FIG. 7 is a schematic representation of the rotation of the polarizationstate of the laser beam 41 (top panel of FIG. 7) which is caused by thefirst and second Pockels cells 61 a, 61 b over a period of time and thepower of radiation which is transmitted by the second polarizer 65 b(bottom panel of FIG. 7) over the same period of time. In the top panelof FIG. 7 the polarization rotation which is caused by the first Pockelscell 61 a is shown with a dashed line and the polarization rotationwhich is caused by the second Pockels cell 61 b is shown with a dottedline.

At the beginning of the time period which is shown in FIG. 7, the firstPockels cell 61 a is in the second mode of operation and rotates thepolarization state of the laser beam 41 by 90°. The second Pockels cell61 b is in the first mode of operation and does not rotate thepolarization state of the laser beam 41. The total rotation of thepolarization state of the laser beam 41 which is caused by the first andsecond Pockels cell 61 a, 61 b is therefore 90° and the laser beam 41 istransmitted by the second polarizer 65 b as can be seen from thetransmitted power shown in the bottom panel of FIG. 7.

At a time t₁ which is shown in FIG. 7 the first Pockels cell 61 a isswitched to the first mode of operation such that it does not rotate thepolarization state of the laser beam 41. At a time t₂ the second Pockelscell 61 b is switched to the second mode of operation such that itrotates the polarization state of the laser beam 41 by 90°. In betweenthe time t₁ and the time t₂ there is a time period t_(i) during whichneither the first Pockels cell 61 a or the second Pockels cell 61 brotates the polarization state of the laser beam 41. The laser beam 41is not transmitted by the second polarizer 65 b during the time periodt_(i) and thus the laser beam 41 is not incident on the photocathode 43.After the time t₂ the second Pockels cell 61 b rotates the polarizationstate of the laser beam 41 by 90° and the laser beam 41 is againtransmitted by the second polarizer 65 b and is incident on thephotocathode 43.

At a time t₃ which is shown in FIG. 7 the first Pockels cell 61 a isswitched back to the second mode of operation such that it rotates thepolarization state of the laser beam 41 by 90°. At a time t₄ the secondPockels cell 61 b is switched to the first mode of operation such thatit doesn't rotate the polarization state of the laser beam 41 by 90°. Inbetween the time t₃ and the time t₄ there is a time period t_(i) duringwhich both the first Pockels cell 61 a and the second Pockels cell 61 brotate the polarization state of the laser beam 41 by 90° to give acombined rotation of 180°. The laser beam 41 is not transmitted by thesecond polarizer 65 b during the time period t_(i) and thus the laserbeam 41 is not incident on the photocathode 43. After the time t₄ thesecond Pockels cell 61 b stops rotating the polarization state of thelaser beam 41 by 90° and the laser beam 41 is again transmitted by thesecond polarizer 65 b and is incident on the photocathode 43.

Also shown in FIG. 7 is a time t₅ when the first Pockels cell 61 a isswitched back to the first mode of operation and a time t₆ when thesecond Pockels cell 61 b is switched back to the second mode ofoperation. There is a time period t_(i) between the time t₅ and the timet₆ during which neither the first Pockels cell 61 a or the secondPockels cell 61 b rotates the polarization state of the laser beam 41and thus the laser beam 41 is not transmitted by the second polarizer 65b.

In the embodiment which is shown in FIG. 7 the first and second Pockelscells 61 a, 61 b are periodically switched between first and secondmodes of operation with time periods T_(a) and T_(b) respectively. Theperiodic switching of the first and second Pockels cells 61 a, 61 b havea phase difference θ with each other which causes the time periods t_(i)during which the laser beam 41 is not transmitted by the secondpolarizer 65 b and therefore the electron beam E is interrupted. It willbe appreciated from FIG. 7 that the length of the time periods t_(i)during which the electron beam E is interrupted is determined by thephase difference θ between the switching of the first and second Pockelscells 61 a, 61 b and not by the time periods T_(a), T_(b) of theswitching. The embodiment of the first control apparatus 51′ which isdepicted in FIG. 6 therefore advantageously allows the laser beam 41 tobe blocked and the electron beam E to be interrupted for time periodst_(i) which are much shorter than the time periods T_(a), T_(b) withwhich the modes of operation of the Pockels cells 61 a, 61 b areswitched. The time periods t_(i) during which the laser beam 41 isblocked and the electron beam E is interrupted may, for example, beapproximately equal to the time period during which one pulse of thelaser beam 41 propagates through the first and second Pockels cells 61a, 61 b. The first control apparatus 51′ of FIG. 6 may therefore be usedto prevent a single pulse of the laser beam 41 from being incident onthe photocathode and may therefore interrupt the electron beam so as tocause a single pulse of the laser beam 41 to have substantially noassociated electron bunch 42 in the electron beam E which is output fromthe injector 21. In some embodiments the first control apparatus 51′ mayinterrupt the electron beam E such that more than one consecutive pulseof the laser beam 41 has substantially no associated electron bunch 42in the electron beam E which is output from the injector 21.

When adjusting the number of pulses of the laser beam 41 which areblocked from being incident on the photocathode 43 in a given time, itmay be desirable to keep the amount of time during which each of thePockels cells are in the first and second modes of operationapproximately constant. This may allow the amount of power from thelaser beam 41 which is dissipated into the Pockels cells to remainapproximately constant and thus the temperature of the Pockels cells mayremain approximately constant. In order to adjust the number of pulsesof the laser beam 41 which are blocked from being incident on thephotocathode 43 in a given time, the phase difference θ between thefirst and second Pockels cells may be adjusted such that the timeperiods t_(i) during which the laser beam 41 is blocked are adjusted.

In an alternative embodiment a half wave plate may be positioned inbetween the first Pockels cell 61 a and the second Pockels cell 61 b.The half wave plate may be configured to rotate the polarization stateof the laser beam 41 by around 90°. In such an embodiment the laser beam41 may be transmitted by the first control apparatus 51′ at times duringwhich both the first and second Pockels cells are in the same mode ofoperation and the laser beam 41 may be blocked by the first controlapparatus 51′ at times during which the first and second Pockels cellsare in different modes of operation.

Embodiments have been described above in which one or more Pockels cells61 are switched between a first mode of operation and a second mode ofoperation. Switching a Pockels cell 61 between a first mode of operationand a second mode of operation may cause some power to be dissipatedinto an electro-optic crystal 62 of the Pockels cell 61. Dissipation ofpower into an electro-optic crystal 61 may cause the electro-opticcrystal 61 to be heated. In some embodiments one or more Pockels cells61 may be cooled in order to stabilize the temperature of the Pockelscells 61.

The first control apparatus 51, 51′ may be operable to control the powerof the laser beam 41 which is incident on the photocathode 43 andthereby control the current of electron bunches which are emitted fromthe photocathode 43. For example, referring to FIGS. 1 and 4 incombination, the power of the laser beam 41 which is transmitted throughthe first control apparatus 51, 51′ may be controlled by the controllerCT in response to the power of the EUV radiation beam B as measured bythe sensor apparatus ST. The first control apparatus 51, 51′ may thusform part of the feedback-based control loop F1. If the measured powerof the EUV radiation beam B is too high then the controller CT willcause the first control apparatus 51, 51′ to block more pulses from thelaser beam 41. Conversely, if the measured power of the EUV radiationbeam B is too low then the controller CT will cause the first controlapparatus 51, 51′ to block less pulses from the laser beam 41.

In an embodiment, the current of the electron beam E may be measuredinstead of the power of the EUV radiation beam. The measured current maybe used to provide a feedback-based control loop instead of the measuredEUV radiation beam power.

In the above described embodiments the voltage applied to thephotocathode 43 may be controlled to accommodate the fact that, due tomissing laser beam pulses, bunches of electrons will not be emitted fromthe photocathode. In general, the voltage applied to the photocathode 43may be stabilized, and this stabilization may take into account missinglaser beam pulses. The voltage stabilization may for example befeed-forward stabilization.

Embodiments of a first control apparatus 51, 51′ have been describedabove which are operable to block one or more pulses of the laser beam41 from being incident on the photocathode 43 so as to interrupt theelectron beam E and to cause at least one pulse of the laser beam 41 tohave substantially no associated electron bunch in the electron beam Ewhich is output from the injector. This may allow the power of theradiation beam B which is output from the free electron laser to becontrolled. In addition to or as an alternative to the use of a firstcontrol apparatus 51, 51′ the power of the radiation beam B which isoutput from the free electron laser may be controlled with a secondcontrol apparatus 52 as shown in FIG. 4. The second control apparatus 52is operable to remove electron bunches 42 which are emitted from thephotocathode 43 from the electron beam E thereby interrupting theelectron beam E so as to cause at least one pulse of the laser beam 41to have substantially no associated electron bunch in the electron beamE.

FIG. 8 is a schematic depiction of an embodiment of the second controlapparatus 52. The second control apparatus 52 comprises a pair ofconducting plates 74 disposed either side of the trajectory of theelectron beam E. The conducting plates 74 are electrically coupled to avoltage source 73 by wires 77. The wires 77, the conducting plates 74and the voltage source 73 form an electric circuit through which acurrent may flow. The direction of the current flow is indicated byarrow heads on the wires 77 in FIG. 8. Also included in the electriccircuit is a resistor 75. Preferably the impedance of the wires 77, theconducting plates 74 and the resistor 75 are matched. The voltage sourceis operable to generate a potential difference between the plates 74 soas to cause a current to flow through the conducting plates. The currentflows in opposite directions through the conducting plates 74 therebygenerating a magnetic field between the plates 74. The voltage source 73is controlled by a controller CT (which may correspond with thecontroller CT shown in FIG. 1). At times at which a potential differenceis generated between the conducting plates 74 so as to create a magneticfield between the plates 74, the generated magnetic field serves toalter the trajectory of electron bunches 42 in the electron beam E so asto direct the electron bunches towards a beam dump 72. An electron bunch42′ is shown in FIG. 8 whose trajectory has been altered by a magneticfield between the conducting plates 74 so as to direct the electronbunch 42′ along a trajectory 71 towards the beam dump 72.

The beam dump 72 comprises a sufficient quantity of material to absorbthe deflected electron bunch 42′. The beam dump 72 absorbs electrons soas to prevent the creation of secondary electrons which maydisadvantageously reach the booster 33 and be accelerated by the linearaccelerator 22. In some embodiments alternative means may be used toprevent creation of secondary electrons. For example, one or moreorifices may be positioned in a beam pipe in which the electron beam Epropagates so as to allow a deflected electron bunch 42′ to propagateout of the beam pipe without creating secondary electrons. In someembodiments secondary electrons may be directed away from the booster 33by, for example, generating an electric and/or a magnetic field which isconfigured to prevent secondary electrons from propagating towards thebooster 33.

The voltage source 73 is operable to switch a potential differencebetween the conducting plates 74 on and off so as to switch between afirst state where no potential difference is generated between theconducting plates 74 and the electron bunches 42 continue along the sametrajectory and remain in the electron beam E and a second state where apotential difference is generated between the conducting plates 74 andone or more electron bunches 42′ are deflected out of the electron beamE and are directed to the beam dump 72. The voltage source 73 may, forexample, be operable to switch between the first and second statessufficiently fast enough so as to only deflect a single electron bunch42′ out of the electron beam E at a time. For example, a first electronbunch 42 may pass through the conducting plates 74 whilst the potentialdifference between the conducting plates 74 is turned off such that thefirst electron bunch 42 is not deflected out of the electron beam E. Apotential difference between the conducting plates 74 may then beswitched on before a subsequent second electron bunch 42′ passes betweenthe conducting plates 74 such that the second electron bunch isdeflected out of the electron beam E and directed along the trajectory71 to the beam dump 72. The potential difference between the conductingplates 74 may then be turned off again before a subsequent thirdelectron bunch 42 passes between the conducting plates 74 such that thethird electron bunch 42 is not deflected out of the electron beam E. Thepotential difference between the conducting plates 74 may, for example,be generated for a time period of less than approximately 10nanoseconds. For example, the potential difference between theconducting plates 74 may be generated for a time period of approximately1 nanosecond or less.

Alternatively a potential difference may be generated between theconducting plates 74 for a sufficient period of time so as to deflectmore than one consecutive electron bunch 42′ out of the electron beam Eand towards the beam dump 72 (e.g. 100 electron bunches or less, e.g. 10electron bunches or less).

An approximation of the angular deflection Δα of an electron bunch 42′which is caused by a potential difference between the conducting plates74 is given by equation 3.

$\begin{matrix}{{\Delta\;\alpha} = {2{\tanh\left( \frac{{\pi\; w}\;}{2h} \right)}\;\frac{qVL}{E_{e}h}}} & (3)\end{matrix}$Where q is the charge of an electron, L is the length along thedirection of propagation of the electron beam E of the conducting plates74, V is the potential difference between the conducting plates 74,E_(e) is the energy of the electrons in the electron bunch 42′, h is theseparation between the conducting plates 74 and w is the width of theconducting plates 74.

In an embodiment a potential difference V may be generated between theconducting plates 74 which results in an angular deflection Δα ofapproximately 1°. This may result in a separation between the deflectedelectron bunch 42′ and the electron beam E of approximately 2 cm at adistance of approximately 1 meter downstream from the conducting plates74. In other embodiments the angular deflection Δα may be greater than1° and may, for example, be as large as 6°. This may result in aseparation between the deflected electron bunch 42′ and the electronbeam E of up to approximately 10 cm at a distance of approximately 1meter from the conducting plates 74.

The potential difference V which is generated between the conductingplates 74 may, for example, be greater than about 0.2 kV. In someembodiments the potential difference V may be as large as 1 kV. Voltagesources 73 which are capable of switching a potential difference of0.2-1 kV on and off sufficiently fast to deflect a single electron bunch42′ out of the electron beam E are commercially available.

As was described above the second control apparatus 52 is operable todeflect one or more electron bunches 42′ out of the electron beam E.Deflected electron bunches 42′ are not therefore accelerated by thebooster 33 or the linear accelerator 22 and do not propagate through theundulator 24. The second control apparatus 52 is thus operable tointerrupt the electron beam E so as to cause at least one pulse of thelaser beam 41 to have substantially no associated electron bunch in theelectron beam E which is output from the injector 21. As was describedabove with reference to the first control apparatus 51 interrupting theelectron beam E causes an interruption in pulses of the radiation beam Bwhich are emitted from the free electron laser FEL and therefore reducesa dose of radiation which is received at a target location of asubstrate W. The second control apparatus 52 may be controlled (e.g. bycontroller CT) in order to control the power of radiation which isemitted by the free electron laser. The controller CT may receive asinput a signal corresponding with the power of the EUV radiation beam(or the current in the electron beam). Thus, a feedback-based controlloop may be provided which allows the power of the EUV radiation beam tobe controlled. This may allow fluctuations of EUV radiation beam powerover an exposure time period to be reduced.

The number of electron bunches 42′ deflected out of the electron beam Emay be sufficiently small that resulting transients in the free electronlaser are relatively low. Thus, for example, the wavelength of the EUVradiation beam output from the free electron laser may remainsubstantially constant. References to a substantially constantwavelength may be interpreted as meaning that the wavelength does notchange or changes by an amount which is sufficiently small that dosevariation at a target location due to wavelength dependent mirrortransmission remains below a desired threshold.

In alternative embodiments one or more electron bunches 42′ may bedeflected out of the electron beam E so as to interrupt the electronbeam E and interrupt pulses in the radiation beam B at other locationsin the free electron laser FEL. For example, one or more electronbunches 42 may be deflected out of the electron beam E after the booster33 and/or after the linear accelerator 22. It is however advantageous todeflect an electron bunch 42′ out of the electron beam E at a time whenthe electron bunch 42′ has a relatively low energy. For example it isadvantageous to deflect an electron bunch 42′ out of the electron beam Ebefore the electron bunch is accelerated by the booster 33 and/or beforethe electron bunch 42′ is accelerated by the linear accelerator 22. Thisis because as the energy of an electron bunch 42 is increased in thebooster 33 and/or the linear accelerator 22, the magnitude of anelectric field which is required to provide a desired deflection of theelectron bunch 42 increases. Deflecting electron bunches 42 which havebeen accelerated by the booster 33 and/or the linear accelerator 22therefore requires a larger potential difference V to be generatedbetween conducting plates 74. Generating a larger potential differencemay take more time and thus the speed with which the potentialdifference may be generated may be increased. This may mean thatdeflecting single electron bunches 42′ out of the electron beam E may beproblematic.

Furthermore an electron bunch which is deflected out of the electronbeam after being accelerated in the booster 33 and/or the linearaccelerator 22 will have a higher energy than an electron bunch 42′which is deflected out of the electron beam before being accelerated. Ahigher energy deflected electron bunch 42′ will generate secondaryelectrons and secondary isotopes which may require removal from the freeelectron laser FEL.

In the embodiment of the second control apparatus 52 which is depictedin FIG. 8 the voltage source 73 causes an electric current to flow inopposite directions in the conducting plates. In an alternativeembodiment one or more voltage sources may be configured to cause acurrent to flow in each of the conducting plates 74 in the samedirection. In such an embodiment a magnetic field may be generatedbetween the plates 74 such that electron bunches 42 which pass betweenthe plates 74 are defocussed by the magnetic field. A defocussedelectron bunch 42 may stimulate emission of radiation in the undulator24 with greatly reduced conversion efficiency when compared to anelectron bunch 42 which has not been defocussed. A pulse of radiationwhich is stimulated in the undulator 24 by a defocussed electron bunchmay therefore have a lower power than a pulse of radiation which isstimulated in the undulator 24 by an electron bunch which has not beendefocussed. An electron bunch 42 may therefore be defocussed in a secondcontrol apparatus in order to reduce the power of a corresponding pulseof radiation which is stimulated in the undulator 24 (e.g. to a level ofpower which is negligible). This may be considered to substantiallyinterrupt pulses in the radiation beam B which is emitted from the freeelectron laser FEL and may therefore cause a reduction of a number ofpulses of radiation which are received by a target location of asubstrate W in a given exposure time period, thereby reducing a dose ofradiation which is received at the target location.

Embodiments of a first control apparatus 51 and a second controlapparatus 52 have been described above which are operable to interruptthe electron beam E so as to cause at least one pulse of the laser beam41 to have substantially no associated electron bunch 42 in the electronbeam E which is output from the injector 21. As was described above,interrupting the electron beam E which is output from the injector 21causes an interruption of pulses in the radiation beam B which isemitted from the free electron laser FEL and therefore causes areduction of a number of pulses of radiation which are received by atarget location of a substrate W in a given exposure time period,thereby reducing the dose of radiation which is received by the targetlocation. The electron beam E may therefore be interrupted by a firstcontrol apparatus 51 and/or by a second control apparatus 52 in order tocontrol the dose of radiation which is received by a target location ofthe substrate W.

The first control apparatus 51 and/or the second control apparatus 52may be controlled (e.g. by the controller CT) in response to one or moremeasurements of the radiation beam B. For example, a radiation sensor(not shown) may be arranged to measure the power of the radiation beam Bwhich is output from the free electron laser FEL. The first controlapparatus 51 and/or the second control apparatus 52 may be controlled inresponse to the measured power of the radiation beam B such that adesired dose of radiation is received by a target location of thesubstrate W. For example, if the radiation sensor measures an increasein the power of the radiation beam B then the first control apparatus 51and/or the second control apparatus 52 may respond to this measurementby interrupting the electron beam E so as to reduce the number of pulsesof radiation which are received by a target location of a substrate W ina given exposure time period such that the dose of radiation which isreceived by the target location continues to be a desired dose despitethe increase in power of the radiation beam B.

In some embodiments measurements of the power of the radiation beam Bmay be measured at other locations in a lithographic system LS than atthe output of the free electron laser FEL. For example, the power of theradiation beam which is output from the beam expanding optics (ifpresent) and which is input to the beam splitting apparatus 19 may bemeasured. Additionally or alternatively the power of one or more branchradiation beams B_(a)-B_(n) may be measured after having been output bythe beam splitting apparatus 19. Additionally or alternatively the powerof one or more branch radiation beams B_(a)-B_(n) may be measured withinone or more lithographic apparatus LA_(a)-LA_(n). For example, the powerof a patterned radiation beam (e.g. patterned radiation beam B_(a)′)which is received at a substrate W may be measured. In general anymeasurements of the radiation beam B or of branch radiation beamsB_(a)-B_(n) may be made and may be used to control the first controlapparatus 51 and/or the second control apparatus 52 so as to control adose of radiation which is received at a target location of a substrateW in a given exposure time period.

In general any measurements of the radiation beam B or of branchradiation beams B_(a)-B_(n) may be made and may be used to control thefirst control apparatus 51 and/or the second control apparatus 52 so asto control the power of radiation which is output from the free electronlaser. The measurement(s) may form part of a feedback-based control loopwhich acts to stabilize the power of the radiation output from the freeelectron laser. An example of a feedback-based control loop F1 is shownin FIG. 1.

In some embodiments the first control apparatus 51 and/or the secondcontrol apparatus 52 may during normal operation interrupt the electronbeam E so as to reduce the number of electron bunches 42 in the electronbeam E and the number of pulses in the radiation beam B by a givenamount. This may allow the first control apparatus 51 and/or the secondcontrol apparatus 52 to stabilize the power of the radiation beam outputby the free electron laser by increasing or decreasing the number ofelectron bunches 42 in the electron beam as needed. This may reducefluctuations of the EUV radiation beam power when that power is averagedover a given time period (e.g. 1 ms).

A target location of a substrate W may be exposed for an exposure timeperiod of approximately 1 ms. The pulse repetition frequency of thelaser beam 41 and therefore the pulse repetition frequency of theradiation beam B may be approximately 300 MHz. Therefore, in thisexample, during an exposure time period of 1 ms a target location of asubstrate W is exposed to approximately 3×10⁵ pulses of radiation. Itmay be desirable to interrupt the electron beam E in a periodic manner.For example, the first control apparatus 51 and/or the second controlapparatus 52 may interrupt the electron beam E such that the every100^(th) or every 1000^(th) pulse of the laser beam 41 has substantiallyno associated electron bunch 42 in the electron beam E. In someembodiments time periods during which the electron beam is interruptedmay in total comprise approximately 1-10% of the time periods duringwhich the electron beam is not interrupted. In some embodiments thefrequency with which the electron beam is interrupted may be 1 MHz ormore (e.g. in the range 1 MHz to approximately 100 MHz). The number ofpulses of the laser beam 41 which have substantially no associatedelectron bunch 42 in the electron beam E during the exposure time periodmay be increased or decreased in order to control the dose of radiationwhich is received during the exposure time period (e.g. to keep dosevariations below a desired threshold). Thus, fluctuations of the EUVradiation beam power averaged over the exposure time period are reduced.Variations in the dose of EUV radiation received at target locations onthe substrate are correspondingly reduced.

In an embodiment in which an exposure slit of radiation is scanned overa target location on a substrate in a scanning exposure it may bedesirable to interrupt the electron beam E in a periodic manner with atime period T_(i) such that the exposure time period T_(e) is an integermultiple of the time period T_(i) (i.e. T_(e)=nT_(i) where n is aninteger). This may ensure that each target location of the substrate Wreceives the same number of pulses of radiation as the exposure slit isscanned over the target location.

As was described above it may be desirable to interrupt the electronbeam E such that only a small number of consecutive electron bunches 42(for example less than 100, or less than 10) are removed from theelectron beam E. For example, it may be desirable to only remove asingle electron bunch 42 from the electron beam E at a time. Removingonly a small number of consecutive electron bunches 42 from the electronbeam E may ensure that there is only a small gap in the electron beam Ewhich propagates through the linear accelerator 22 and the undulator 24.This will reduce the size of transients which will arise in the freeelectron laser due to the gap in the electron beam E (compared with if abigger gap were present). The transients may be sufficiently small thatfor example the wavelength of EUV radiation generated by the freeelectron laser remains substantially constant. References to asubstantially constant wavelength may be interpreted as meaning that thewavelength does not change or changes by an amount which is sufficientlysmall that dose variation at a target location due to wavelengthdependent mirror transmission remains below a desired threshold.

The linear accelerator 22 may comprise a plurality of resonant cavities(e.g. superconducting radio frequency cavities) in which externallydriven oscillating electromagnetic fields resonate. Each electron of anelectron bunch 42 which passes through a resonant cavity has its ownassociated electromagnetic field. As an electron passes through a cavityits electromagnetic field is perturbed causing an electromagnetic fieldknown as wakefield to exist within the cavity. The electromagnetic fieldin a cavity at a given time is therefore a combination of an externallydriven electromagnetic field and the wakefields of electrons which havepreviously passed through the cavity. An electron bunch 42 whichimmediately follows an interruption in the electron beam E may thereforeexperience a different electromagnetic field in cavities of the linearaccelerator 22 than an electron bunch 42 which does not follow aninterruption in the electron beam E. An electron bunch 42 whichimmediately follows an interruption in the electron beam E may thereforebe accelerated by a different amount in the linear accelerator 22 thanan electron bunch 42 which does not follow an interruption in theelectron beam E and may therefore have a different energy in theundulator 24.

In some embodiments the electron beam E which is output from theundulator 24 is passed back through the linear accelerator 22 in orderto recover energy from the electron beam E and to decelerate theelectron beam E. Such an arrangement is known as an energy recoveringlinear accelerator (ERL). The energy which is recovered from thedecelerated electrons is used to accelerate the electron beam E which isoutput from the injector 21. If the electron beam E is interrupted byremoving an electron bunch 42 from the electron beam E then there willbe an interruption in the recovery of energy from the electron beam inthe linear accelerator 22. Such an interruption in the recovery ofenergy may cause a reduction in the amount by which acceleratingelectrons passing through the linear accelerator 22 are accelerated andmay therefore change the energy of some electron bunches 42 which areoutput from the linear accelerator 22 and which propagate through theundulator 24.

As was described above the effect of interrupting wakefields in cavitiesof the linear accelerator 22 and/or the effect of interrupting therecovery of energy from decelerating electrons in the linear accelerator22 may cause some electron bunches 42 to pass through the undulator 24with a different energy to electron bunches 42 which are not affected bythese effects. The wavelength of radiation whose emission is stimulatedin the undulator 24 is dependent on the energy of the electron bunchesin the undulator 24 and thus variations in the energy of electronbunches in the undulator 24 may lead to variations in the wavelength ofthe radiation beam B which is emitted from the free electron laser FEL.An interruption in the electron beam E may therefore cause a variationin the wavelength of pulses of the radiation beam B which immediatelyfollow the interruption in the electron beam E.

The transmittance of the optical path which the radiation beam B followsfrom the free electron laser FEL to a substrate W may be dependent onthe wavelength of the radiation beam B. Variations in the wavelength ofthe radiation beam B may therefore cause variations in the power of aradiation beam (e.g. patterned radiation beam B_(a)′) which is incidenton a substrate and may therefore affect the dose of radiation which isreceived at the substrate W. It may be desirable to reduce any impactthat interruptions in the electron beam E have on the wavelength of theradiation beam B.

A change in the wavelength of the radiation beam B which is caused by aninterruption in the electron beam E may depend on the length of time forwhich the electron beam E is interrupted. For example, removing only asingle electron bunch 42 from the electron beam E may cause the electronbeam E to be interrupted for only a short amount of time and may causeonly a small variation in the wavelength of pulses of the radiation beamB which follow the interruption. If the number of consecutive electronbunches 42 which are removed from the electron beam E is increased thenthis will increase the amount of time for which the electron beam E isinterrupted and may therefore increase the variation in the wavelengthof pulses of the radiation beam B which follow the interruption. It maytherefore be desirable to interrupt the electron beam E such that only alimited number of consecutive electron bunches 42 are removed from theelectron beam E in order to reduce the impact of the interruption on thewavelength of the radiation beam B. For example, it may be desirable toonly remove a single electron bunch 42 from the electron beam E at atime. In practice this may not be possible due to the high frequency ofthe electron pulses (e.g. around 100 MHz). In general, it may bedesirable to interrupt the electron beam with a repetition rate of 1 MHzor more, and to limit total interruptions of the electron beam to lessthan 10% of the beam time. This will limit the extent to which theinterruptions cause variations of the wavelength of pulses in theradiation B. For example 100 consecutive electron bunches or less may beremoved from the electron beam at a time.

In an embodiment, transients which are caused by removing a plurality ofelectron bunches 42 from the electron beam E may be measured and takeninto account. For example, transient changes of the wavelength of theradiation beam may be measured and taken into account. As noted above,the transmission of mirrors of the lithographic apparatus LA iswavelength dependent. The collective transmission of the mirrors has apeak transmission wavelength, with the transmission of the mirrors beinghighest at that peak and falling off along a slope either side of thepeak as a function of wavelength. The free electron laser may beconfigured to operate at a wavelength which lies on a slope of thecollective mirror transmission (e.g. around a mid-point of the slope).When a plurality of electron bunches are removed from the electron beamE the accelerator 21 retains energy which would otherwise have beentransferred to the electron bunches. When the next electron bunches passthrough the accelerator 21 they receive this additional energy. This isa transient effect which may last for around 1 μs. Since the electronbunches have additional energy they will generate EUV radiation with ashorter wavelength. The collective transmission of the mirrors at thisshorter wavelength will be different (e.g. lower), and as a result theEUV radiation will be attenuated more by the mirrors before it isincident upon a substrate. This effect may be used to compensate, atleast in part for the effect of transients caused by removing aplurality of electron bunches 42 from the electron beam (or defocusingthe electron bunches in the manner described above).

In a related embodiment, the amount of charge in the electron bunches 42may be altered by changing the power of the pulses of the laser beam 41.This will yield an immediate change in the intensity of EUV radiationemitted by those electron bunches 42 and the wavelength of the EUVradiation. In addition, it will create a transient change of the energyin the accelerator 21 which will influence the energy and wavelength ofsubsequent electron bunches 42 (e.g. for a period of a few μs). Theseenergy and wavelength changes may be used, in combination with thewavelength dependent collective transmission of the mirrors, to controla dose of EUV radiation delivered to a target location on a substrate W.

In a further related embodiment the wavelength of the EUV radiation beammay be adjusted by changing the acceleration of the electron bunches 42.As explained above, changing the wavelength may be used as a way ofcontrolling the dose of radiation delivered to a target location on asubstrate W. The acceleration provided to the electron bunches 42 may bechanged using an extra cavity (e.g. formed from copper) in the electronbooster 33 or the accelerator 21. The extra cavity may have a peakelectric field significantly smaller than electric fields used by othercavities but large enough to be able to control the wavelength of theresulting EUV radiation pulses. If the extra cavity is notsuperconducting (e.g. is formed from copper) then it will have a smallerQ factor and will be able to control the wavelength more rapidly.

The free electron laser FEL may include one or more safety monitoringsystems which monitor properties of the free electron laser FEL. Forexample, the current of electron bunches 42 which are output from theinjector 21, the electron beam E which propagates through the freeelectron laser FEL and/or pulses of the radiation beam B may bemonitored by one or more safety monitoring systems. One or more safetysystems may act upon measurements made by a safety monitoring system inorder to regulate a monitored variable and/or to provide an error alertin the event that a monitored variable differs from a desired state. Forexample, in the event that pulses of the radiation beam B and/orelectron bunches 42 are missing this may be detected by one or moresafety monitoring systems and may be acted upon by a safety system (e.g.by shutting down the free electron laser FEL). In the event that firstcontrol system 51 and/or the second control system 52 interrupts theelectron beam E then this interruption may be communicated to the safetymonitoring system and/or the safety system such that a missing electronbunch E and/or a missing pulse in the radiation beam B is not acted uponby the safety system.

Advantages of interrupting an electron beam E have been described abovein the context of controlling a dose of radiation which is received by atarget location of a substrate W. Interrupting an electron beam E may beadditionally advantageous since it may allow ions which may congregatealong the path of the electron beam E to dissipate away from the path ofthe electron beam E. Ions may be created in a beam pipe through whichthe electron beam E propagates in the free electron laser FEL and may beattracted to the path of the electron beam E by the potential well whichis caused by the electrons. Electrons in the electron beam E may bescattered by the ions and may create harmful radiation. Scatteredelectrons may additionally cause damage to magnets in the undulator 24.An interruption in the electron beam E causes an interruption in thepotential well which is created by the electron beam E during which theions may dissipate away from the path of the electron beam E. Dissipatedions may, for example, be absorbed by the walls of a beam pipe such thatthey are removed from the beam pipe.

Reference has been made above to the removal of one or more electronbunches 42 from the electron beam E. Removal of one or more electronbunches 42 from the electron beam E may comprise substantiallypreventing one or more pulses of the laser beam 41 from being incidenton the photocathode 43 such that substantially no electron bunch 42 isemitted from photocathode 43 (e.g. using the first control apparatus51). Removal of one or more electron bunches 42 from the electron beam Emay comprise reducing the power of one or more pulses of the laser beam41 incident on the photocathode 43 to less than 10% of the nominal powerand thereby reducing the charge of electron bunches 42 emitted fromphotocathode 43 such that they produce a negligible amount of EUVradiation (e.g. using the first control apparatus 51). Removal of one ormore electron bunches 42 from the electron beam E may comprisedeflecting one or more electron bunches 42 from the electron beam E(e.g. using the second control apparatus 52).

In an alternative approach, as explained above, instead of removing oneor more electron bunches 42 from the electron beam E, one or moreelectron bunches may be defocussed such that they produce a negligibleamount of EUV radiation (e.g. using the second control apparatus 52)

An alternative embodiment of the invention will now be described withreference to FIG. 9. FIG. 9 shows an FEL which has many features incommon with the FEL shown in FIG. 3. To avoid repetition these featuresare not described again here. The free electron laser FEL comprises aradiation sensor 25, which is operable to determine the power of theradiation beam B. A portion of the radiation beam B may be directedtowards the radiation sensor apparatus ST (which may correspond with theradiation sensor apparatus ST shown in FIG. 1), and the irradiance ofthat portion may be measured. This may be used to determine the power ofthe beam B. Alternatively, or additionally, the power of the beam B maybe determined indirectly. For example, residual gas present in thevacuum pipe through which the radiation beam B passes may fluoresce(absorbing EUV radiation and emitting radiation with a differentwavelength) and/or cause Rayleigh scattering of the EUV radiation. Ameasurement of fluorescence and/or Rayleigh scattering by such residualgas, together with a measurement of residual gas pressure, may besufficient to determine the power of the beam B. The measurement of thepower may be substantially continuous or intermittent.

The undulator 24 has an adjustment mechanism 24 a that is operable, inresponse to the power determined by the radiation sensor apparatus ST,to vary one or more parameters of the undulator such that the power ofthe radiation beam is altered as will now be described in furtherdetail.

As electrons move through the undulator 24, they interact with theelectric field of the radiation, exchanging energy with the radiation.An electron which meets the resonance condition as it enters theundulator will lose energy as radiation is emitted, so that theresonance condition is no longer satisfied. Therefore, as explainedabove, in some embodiments the undulator may be tapered. That is, theamplitude of the periodic magnetic field and/or the undulator periodλ_(u) may vary along the length of the undulator in order to keepbunches of electrons at resonance as they are guided though theundulator. Advantageously, tapering of the undulator has the capacity tosignificantly increase conversion efficiency. The use of a taperedundulator may increase the conversion efficiency (i.e. the portion ofthe energy of the electron beam E which is converted to radiation in theradiation beam B) by more than a factor of 2. The tapering of theundulator may be achieved by reducing the undulator parameter K alongits length.

The undulator period λ_(u) and the magnetic field strength B₀ along theaxis of the undulator may be matched to the electron bunch energy tohelp ensure that the resonance condition is met. In the case of anuntapered undulator, the undulator period λ_(u) and the amplitude of theperiodic magnetic field B₀ remain constant throughout the undulator. Inthe case of a tapered undulator, the undulator period λ_(u) and/or theamplitude of the periodic magnetic field B₀ varies with distance alongthe axis of the undulator so as to help ensure that the resonancecondition is met. This matching provides maximum or increased energyextraction from the electrons to EUV radiation for a given length ofundulator. This matching of the undulator period λ_(u) and the magneticfield strength B₀ to help ensure that the resonance condition is met maybe a default configuration of the free electron laser FEL.

In an embodiment, at least part of the magnetic field of the undulator24 is adjustable, the adjustment mechanism 24 a being operable to varyit. When required, the strength of at least part of the magnetic fieldin the undulator can be dynamically changed in order to reduceconversion efficiency from the above described matched configuration. Insome embodiments, the adjustment mechanism 24 a is operable to alter themagnetic field strength on, or close to, the axis of the undulator 24.

Advantageously, this provides control of the power of the output EUVradiation while one or more properties of the electrons such as, forexample, energy, charge, compression, focusing and repetition rate mayremain constant. Such an arrangement is beneficial for, for example, oneor more reasons. For example, such an arrangement allows for changes indemand for the EUV radiation to be accounted for. For example, such anarrangement allows varying requirements of the lithographic apparatus LAthat uses the EUV radiation to be accommodated. The lithographicapparatus LA comprises several mirrors, which may deteriorate over timeand the arrangement, for example, allows such deterioration to becompensated for. As another example, such an arrangement allows fordegradation of the magnets in the undulator 24 due to electron and/orneutron bombardment to be compensated for.

A further benefit is that the conversion efficiency of the FEL canchange relatively quickly. In particular the power of the radiation beamB may be changed within the timescale for exposure of a target locationon a substrate W by the lithographic apparatus LA (e.g. within around 1ms). This may allow the power of the radiation beam B to be controlledsuch that the dose of radiation received at a target location on asubstrate is controlled. That is, feedback from the sensor apparatus STmay be used to reduce fluctuations in the dose of radiation received atthe target location (e.g. such that variation of the dose is kept belowa desired threshold).

Depending on the repetition rate of the bunched electron beam E, it maynot be possible to correct pulse to pulse variations in power.

The magnetic field strength on, or close to, the axis of the undulator24 may be altered by moving the magnets which provide the magnetic fieldof the undulator 24 towards or away from the beam axis. This may beconsidered to be an altering of the tapering of the undulator 24. Themagnets may be adjusted independently or in dependence on the othermagnets. In an embodiment, the magnets are moved relative to the beamaxis in such a way that the polarization of the EUV radiation remainssubstantially unaltered. This may be advantageous if it is desired orrequired to provide the lithographic apparatus LA with radiation of aspecific polarization. For example, the lithographic apparatus LA mayrequire circularly polarized radiation. Adjustment of the tapering of aplanar undulator 24 by moving the magnets will have substantially noeffect on polarization. However, adjustment of the magnets of a helicalundulator will alter the polarization of the radiation unless theadjustment of magnets is in both the vertical and horizontal directions.

In an embodiment, magnets of the undulator 24 may be arranged on eitherside of the beam axis with a gap between them. The gap may for examplebe between 4 and 10 mm. The size of the gap between magnets may forexample be controlled to an accuracy of around 1 micron. The adjustmentmechanism 24 a may be operable to change the size of the gap by forexample 10 microns or more.

Additionally or alternatively, the magnetic field strength on, or closeto, the axis of the undulator 24 may be altered by altering the magneticfield produced by the magnets. The magnets may be permanent magnets andthe magnetic field that they produce may be altered by altering acurrent flowing through coils positioned next to the magnets and/or thetemperature of the magnets. An increase in the temperature of themagnets may cause a decrease in the magnetic field strength.

In an embodiment, the adjustment mechanism 24 a is operable to alter theundulator period λ_(u) of the undulator 24.

In an embodiment, the radiation sensor apparatus ST can be used tomonitor the power of the radiation beam B. In response to thisdetermined power, the adjustment mechanism 24 a may alter the periodicmagnetic field of the undulator 24, which, in turn, will alter the powerof the radiation beam B. In this manner a feedback-based control loopmay be established which controls the power of the radiation beam Boutput by the free electron laser.

The adjustment mechanism 24 a may by controlled by a controller CT. Theradiation sensor apparatus ST may be connected to the controller CT andmay be operable to send a signal S to the controller indicative of thedetermined power. The connection between the radiation sensor ST and thecontroller may physical or wireless (for this or other embodiments). Thecontroller CT may be operable to receive the signal S and, in responsethereto, alter one or more parameters of the undulator 24. The parametermay be altered in dependence on the signal S received from the radiationsensor 25 according to a certain (e.g. predetermined) algorithm. Thesensor apparatus ST, controller CT and adjustment mechanism 24 acomprise a feedback-based control loop which may be used to control thepower of the radiation beam output by the free electron laser (e.g. toreduce power fluctuations).

The controller CT may be operable to receive an additional signal S′and, in response thereto, alter one or more parameters of the undulator24. For example, the processor may receive a signal S′ from alithographic apparatus LA indicating an anticipated period of low orhigh demand for EUV radiation and the processor may alter one or moreparameters of the undulator 24 accordingly.

The free electron laser FEL may comprise a first deflecting magnetdisposed between the accelerator 22 and the undulator 24, which can bein: a first state wherein the electrons are guided along the periodicpath by the undulator 24 such that they interact with radiation in theundulator 24, stimulating emission of coherent radiation; or a secondstate wherein the electrons are guided along a different path throughthe undulator 24 such that they decouple from radiation in the undulator24 and substantially no emission of coherent radiation is stimulated.For example, the first deflecting magnet may be switched on (wherein itis in the second state) or off (wherein it is in the first state). Thisarrangement is particularly applicable to helical undulators wherein asmall change in the angle between the direction of the electrons and theaxis of the undulator (for example by around 10 μrad) can cause completedecoupling of the EUV radiation from the electron bunches. Thereforestimulated emission of EUV radiation is effectively switched off,reducing EUV output of the free electron laser FEL to negligible levels.Advantageously, this provides an emergency switch off feature which maybe desirable. The free electron laser FEL may comprise a seconddeflecting magnet disposed downstream of the undulator 24, which isarranged to compensate for the action of the first deflecting magnet sothat electrons exiting the second deflecting magnet when the firstdeflecting magnet is in the on state follow substantially the same thetrajectory as electrons exiting the second deflecting magnet when thefirst deflecting magnet is in the off state. This allows the electronsto be directed towards the beam dump 100 when the first deflectingmagnet is on or off.

The typical divergence of EUV radiation from a SASE FEL is in the orderof at least several tens of μrad. Therefore, in principle, it ispossible by controlling the beam direction at the input of undulator tochange the power of EUV output without significantly changing theposition of the radiation beam B exiting the free electron laser FEL. Adeflection of the electron beam E can be achieved dynamically usingdeflecting magnets (driven by pulsed current), by mechanical movement ofbending magnets and/or by changing a current through the coils of one ormore dipole magnets.

As noted above, the free electron laser FEL may comprise a decelerationmechanism 26 disposed between the undulator 24 and the beam dump 100,which is operable to reduce the energy of the electrons before they areabsorbed by the beam dump 100. Such an arrangement reduces the amount ofenergy the electrons have when absorbed by the beam dump and, therefore,will reduce the levels of induced radiation and secondary particlesproduced. This removes, or at least reduces, the need to remove anddispose of radioactive waste from the beam dump. This is advantageoussince the removal of radioactive waste requires the free electron laserFEL to be shut down periodically and the disposal of radioactive wastecan be costly and can have serious environmental implications.

The deceleration mechanism 26 may be operable to reduce the energy ofthe electrons to below 7 MeV and, desirably, below 5 MeV.Advantageously, electrons below this energy do not induce anysignificant level of radioactivity in the beam dump 100. Duringoperation of the free electron laser FEL, gamma radiation will bepresent but when the electron beam E is switched off, the beam dump 100will be safe to handle.

One known deceleration mechanism uses a linear accelerator to deceleratethe electrons. For example, the linear accelerator 22 that is foracceleration may also be used for deceleration, by injecting theelectron bunches E that leave the undulator 24 into the linearaccelerator 22 with a phase difference of 180 degrees relative to theradio frequency (RF) field. Such an arrangement is known as an energyrecovering LINAC. However, there is a limit to the spread of electronenergies within the bunches that such an arrangement can accept. Theundulator 24 will introduce a spread in the energy of the electron beamE as it passes through. This will result in imperfect deceleration ofelectron bunches that are injected into the linear accelerator 22 with aphase difference of 180 degrees relative to the radio frequency (RF)field. As a result, some of the electrons may have a larger energy asthey leave the linear accelerator 22 than that of electrons leaving theinjector 21. Therefore some of these electrons may have energies inexcess of the desired threshold of 7 or 5 MeV. As a result, a mechanismto further reduce the energy of these electrons may be required.

Therefore, at least part of the deceleration mechanism 26 may beseparate from the electron source. In particular, the decelerationmechanism 26 may comprise a synchrotron or a cyclotron, which may beused to decelerate the electrons actively. Advantageously, such anarrangement allows for greater spreads of electron energies within thebunches leaving the undulator, in turn allowing an increase in theefficiency of the free electron laser. In an alternative embodiment, thedeceleration mechanism 26 may comprise a conductive conduit which theelectrons pass through to passively decelerate them. For example,referring to FIGS. 11A and 11B, the deceleration mechanism 26 maycomprise a section of conductive piping 140 a, 140 b with a rough innersurface to promote energy dissipation due to wake fields. For example,the inner surface may comprise a plurality alternating recesses 141 a,141 b and protrusions 142 a, 142 b. The alternating recesses 141 a, 141b and protrusions 142 a, 142 b may have any suitable shape in profilesuch as, for example, triangular (141 a, 142 a) or rectangular (141 b,142 b). The inner surface of the conductive piping may contain anysuitable source of discontinuities for inducing large wake fields suchas holes, slits, etc as appropriate. The conductive piping 140 a, 140 bmay comprise a cooling system (not shown) such as water cooling.

In an embodiment, the adjustment mechanism 24 a is operable to vary thepolarization of the radiation beam B. This may be in response to asignal S′ received by the adjustment mechanism. As will be describedfurther below, the polarization of the output radiation beam B may bealtered by (a) changing the geometry of the undulator 24; and/or (b)manipulating the radiation beam B leaving the undulator 24, for example,using a system of mirrors (see FIG. 10).

In one embodiment, the radiation beam B produced by the free electronlaser is guided into the illumination system IL of the lithographicapparatus LA and to the patterning device MA. In general, the radiationwill change direction between the free electron laser FEL and thepatterning device MA, the change in direction being achieved using oneor more mirrors. The one or more mirrors may include the facetted fieldmirror device 10 and facetted pupil mirror device 11 in the illuminationsystem IL and/or any other mirror positioned between the free electronlaser FEL and the lithographic apparatus LA and/or in the illuminationsystem IL. In general, each reflection will alter a ratio of intensitiesof components wherein the electric field is parallel or perpendicular tothe plane of incidence (often referred to as s- and p-components of theradiation), changing the polarization of the radiation. The change inthe polarization is dependent upon the angle of incidence with a grazingincidence mirror producing the greatest change and a near normalincidence mirror (such as the facetted field mirror device 10 and/orfacetted pupil mirror device 11 in the illumination system IL) notproducing any significant change in the polarization.

The free electron laser FEL may be configured so that the polarizationof the radiation beam B is chosen in dependence on the one or moremirrors disposed between the free electron laser FEL and the patterningdevice MA, such that the radiation incident upon the patterning devicehas a desired polarization. For example, it may be desirable toirradiate the patterning device with circularly polarized radiation. Ifthis is the case then the undulator 24 may produce ellipticallypolarized radiation with the relative intensities of s- and p-componentsbeing such that the radiation incident upon the patterning device MA iscircularly polarized. The relative intensities of s- and p-componentsmay be chosen by only taking into account any grazing incidence mirrorsin the optical path, and neglecting the effect of any normal or nearnormal incidence mirrors.

In order to produce elliptically polarized radiation, any suitableundulator 24 may be used such as a helical undulator. In an embodiment,the undulator 24 may comprise two coaxial planar undulators whose planesare substantially mutually perpendicular wherein the lengths, undulatorperiods and magnetic field strengths of the two planar undulators arechosen to help ensure that the ratio of horizontal and verticalintensities of polarization produces the desired elliptical radiation.In an embodiment, the undulator may comprise more than two coaxialplanar undulators, the planes of each planar undulator being different.Such an arrangement may allow for smoother polarization matching than anarrangement using only two planar undulators. In an embodiment, theundulator may comprise a first and second coaxial planar undulatorswhose planes are substantially mutually perpendicular and a coaxialhelical undulator disposed between the first and second planarundulators. Such an arrangement is advantageous because it allows forefficient coupling of energy from a first polarization radiation (fromthe first planar undulator) into the radiation of a substantiallyperpendicular polarization (corresponding to the second planarundulator) by introduction of the middle helical undulator.

The lithographic apparatus LA may require circularly polarized EUVradiation. This may be achieved using a helical undulator, whichgenerates circularly polarized radiation. However, if the undulator isplanar then conversion from linear to circular polarization may berequired. Optionally, this may be done using the apparatus 130, whichwill now be described further with reference to FIG. 10.

The apparatus 130 is operable to split the radiation beam B into twocomponents using a mirror 131. A first portion B1 of the radiation beamB is reflected through 90° by the mirror 131, a second portion B2 of theradiation beam B continuing past the mirror 131. The polarization vectorof the radiation beam B is parallel to the mirror 131 and therefore thepolarization vector of the first portion B1 is the same as that of theradiation beam B. The first portion B1 then undergoes two subsequentreflections through 90° by mirrors 132, 133. The second reflection, fromthe mirror 132, rotates the polarization vector of the first portion B1by 90°, whereas the third reflection, from the mirror 133, does not.After the three rotations through 90°, the first portion B1 is parallelto, but offset from the second portion B2 and the polarization vectorsof the two portions B1, B2 are mutually orthogonal. Two grazingincidence mirrors 134, 135 are used to guide the first portion B1 sothat it converges with the second portion B2. Although the first andsecond portions B1, B2 converge, as a result of the reflections of thefirst portion B1, the polarization vectors of the first and secondportions B1, B2 are no longer parallel. By taking into account thedifferent reflective losses of the first and second portions B1, B2 andchoosing the power of the first and second portions B1, B2appropriately, it is possible to produce two converging beams ofsubstantially equal power and substantially mutually orthogonalpolarization. These two beams may together, via appropriate selection oftheir relative phase, form a circularly or elliptically polarizedradiation beam. In practice, precise phase matching may not be possible.Therefore, alternatively, the co-propagating or overlapping beams B1 andB2 may both be received by the lithographic apparatus LA and projectedonto the substrate W (having been patterned by the mask MA). Theapparatus 130 may be used to ensure that both polarizations are presentduring exposure of the substrate W and that the doses delivered by thetwo polarizations are, on average, approximately equal.

The bunch compressor of the free electron laser may comprise anadjustable bunch compressor. FIG. 12 illustrates a free electron laserFEL with an adjustable bunch compressor 230. The adjustable bunchcompressor 230 may be arranged to adjustably control at least one of:(i) a charge density distribution of the electron bunches of theelectron beam E along its direction of propagation before they enter theundulator 24; or (ii) an average energy of the electron bunches in theelectron beam E before they enter the undulator 24. The adjustable bunchcompressor 230 comprises a resonant cavity 232 and a magnetic compressor234. The resonant cavity 232 is disposed ‘up stream’ of the magneticcompressor 234, i.e. the electron beam E passes first through theresonant cavity 232 and then through the magnetic compressor 234.

In the arrangement of FIG. 12, the linear accelerator 22 comprises aplurality of superconducting radio frequency cavities 222 a, which areaxially spaced along a common axis. These are supplied withelectromagnetic radiation by one or more radio frequency power sourcesso as to excite an oscillating electromagnetic standing wave within thecavities 222 a. The radio frequency power source comprises a low powerradio frequency source 225 and a high power amplifier 222 b. Theelectromagnetic energy is communicated to the superconducting radiofrequency cavities 222 a via a waveguide 222 c. A frequency at which theelectric field along the common axis oscillates is chosen tosubstantially match that of the electron beam E. The timing is such thatas each bunch of electrons passes through each cavity, the electricfield along the common axis accelerates the electrons.

As each bunch of the electron beam E passes through the linearaccelerator 22, the electrons in different parts of the bunch will, ingeneral, experience different accelerating forces due to the length ofeach bunch. For example, electrons towards the front of the bunch willexperience a different accelerating force to those towards the rear ofthe bunch since the electromagnetic standing wave within thesuperconducting resonant cavities at a given point on the common axiswill change in the time taken for the electron bunch to traverse thatpoint. Therefore, in addition to accelerating the electron beam, thelinear accelerator 22 will also introduce a correlation between theenergy of the electrons and their position within the bunch. Such anenergy-position correlation is known as a ‘chirp’ of the electron bunch.

By convention, if the energy of the individual electrons increasestowards the front (rear) of the electron bunch then the chirp of theelectron beam may be said to be positive (negative). The chirp of theelectron bunches in the electron beam E may be positive or negativedepending on whether the electron bunches are accelerated (ordecelerated) on the rising or falling slope of the radio frequencyelectromagnetic wave. Although the radio frequency standing wave may besinusoidal, for relatively short electron bunches, wherein the timetaken for the electron bunch to traverse a given point on the commonaxis is relatively short, the chirp of the electron beam E may besubstantially linear. For longer electron bunches, the chirp may not belinear.

The magnetic compressor 234 is arranged to compress the electron bunchesin the electron beam E along a direction of propagation of the electronbeam. Further, the compression is dependent on the chirp of the electronbunches as they enter the magnetic compressor. For example, the magneticcompressor 234 may comprise a plurality of magnets arranged to spreadeach electron bunch and subsequently recombine it so that the length ofa path followed by each electron as it passes through the magneticcompressor 234 is dependent upon its energy. Such an arrangement can beused to use a given chirp to compress the beam. For example, themagnetic compressor 234 may be arranged such that for an electron bunchwith a negative chirp (i.e. the electrons towards the rear of the bunchhave more energy than those towards the front of the bunch) higherenergy electrons within each bunch follow a shorter path than lowerenergy electrons.

Altering the charge density distribution of the electrons in theelectron bunches along their propagation direction will alter a gain ofthe undulator 24 (the conversion efficiency is dependent on the peakcurrent of the electron bunches). In turn, this will alter the power ofthe radiation beam B output by the undulator 24.

Under steady operating conditions, the chirp introduced to electronbunches of the electron beam E as it is accelerated by the linearaccelerator 22 may be substantially constant. By either: (a) alteringthis chirp slightly before the electron beam E enters the magneticcompressor 234; or (b) altering the magnetic compressor 234 such that itapplies a different compression for a given chirp, the charge densitydistribution of the electron beam along its propagation direction can becontrolled before it enters the undulator 24. In turn, this provides amechanism to control the power of the radiation beam B output by theundulator 24. The adjustable compressor 230 is operable to control thepower of the radiation beam B by altering the chirp of the electronbunches in the beam E slightly before they enter the (passive) magneticcompressor 234. Therefore, advantageously, the arrangement of FIG. 12provides a free electron laser whose output power can be activelycontrolled.

The magnetic compressor 234 is passive and remains fixed. The resonantcavity 232 is arranged to control the chirp of the electron beam beforeit enters the magnetic compressor 234. By using the resonant cavity 232to increase or decrease the chirp of the electron bunches in theelectron beam E the power of the radiation beam B output by theundulator 24 can be controlled.

The resonant cavity 232 is provided separate from the linear accelerator22. The resonant cavity 232 is ‘phase locked’ with the linearaccelerator 22, i.e. the resonant cavity 232 operates at substantiallythe same frequency as the linear accelerator 22 and is arranged suchthat a phase of the resonant cavity 232 with respect to the electronbeam E remains substantially constant. This may be achieved by using thesame low power radio frequency power source 225 to supplyelectromagnetic energy to the linear accelerator 22 and the resonantcavity 232. The resonant cavity 232 is provided with its own amplifier236 and electromagnetic energy is communicated from the amplifier 236 tothe resonant cavity 232 via a waveguide 238.

The free electron laser of FIG. 12 further comprises a controller CT.The controller CT is operable to receive an input signal S from a sensorapparatus ST which measures the power of the EUV radiation beam B outputfrom the undulator. In response to the signal S, the controller CT isoperable to adjust the electromagnetic power that is supplied to theresonant cavity 232 via the amplifier 236 and waveguide 238. Thus, afeedback-based control loop is provided which comprises the sensorapparatus ST, the controller CT and the adjustable compressor 230. Thefeedback-based control loop may be used to reduce fluctuations in thepower of the EUV radiation beam (e.g. when the power is averaged over atime period such as around 1 ms). The feedback-based control loop may beused to keep variations of EUV radiation dose incident upon substratetarget locations below a desired threshold.

As described above, the adjustable compressor 230 of FIG. 12 may be usedto control the power of the radiation beam B by altering the chirp ofthe electron bunches in the electron beam E slightly before they enterthe (passive) magnetic compressor 234. Alternatively or in addition, theadjustable compressor 230 of FIG. 12 may be used to control an averageenergy of the electron beam E before it enters the undulator 24.

A relatively small alteration to the mean energy of the electron bunchesin the electron beam E before they enter the undulator 24 will result inan alteration of the wavelength of the output radiation beam B. In turn,this will also result in an alteration of the power of the outputradiation beam B. This is a result of two factors: (i) a small change inthe average energy of each bunch will affect the gain of the freeelectron laser; and (ii) the energy of each photon produced in theundulator 24 is dependent upon its wavelength. Therefore,advantageously, such an arrangement provides a free electron laser whoseoutput power and wavelength can be actively controlled.

The free electron laser of FIG. 12 may form part of the lithographicsystem LS of FIG. 1, wherein radiation produced by the free electronlaser is ultimately received by one or more substrates W within one ormore lithographic apparatuses LAa-LAn. These substrates may be exposedusing scanning exposures, wherein a given target location on a substrateW is illuminated by EUV radiation for a predetermined period of time(e.g. around 1 ms). Within the lithographic system LS, radiation istransported from the free electron laser to the substrates via: (i) abeam delivery system (for example comprising a beam splitting apparatus19); and (ii) optics within the lithographic apparatuses LAa-LAn (forexample optics 10, 11, 13, 14—see FIG. 2). For thermal reasons, theoptics within the beam delivery system may comprise mainly grazingincidence mirrors and, as such, a combined reflectivity of these opticsmay be relatively independent of the wavelength of the radiation beam B.In contrast, optics within the lithographic apparatuses LAa-LAn maycomprise near normal incidence mirrors, and may comprise multilayermirrors that are optimized for a given nominal wavelength. As such, acombined reflectivity of the optics within the lithographic apparatusesLAa-LAn may be strongly dependent on the wavelength, and bandwidth, ofthe radiation beam B.

A change in the output power of the radiation beam B will directlyaffect a dose of radiation delivered by the free electron laser to atarget location on a substrate W. Furthermore, for the reasons explainedabove, altering the wavelength of the radiation beam B will affect thedose of radiation delivered to a target location on the substrate. Thechange in wavelength of the radiation beam may have a greater effect onthe dose delivered to the target location by the radiation source thanthe change in the power of the radiation beam. The feedback-basedcontrol loop comprising the sensor apparatus ST, the controller CT andthe adjustable compressor 230 may be used to reduce fluctuations in thepower and wavelength of the radiation beam B. This may in turn keepvariations of EUV radiation dose incident upon substrate targetlocations below a desired threshold.

The adjustable compressor 230 may operate in a plurality of differentmodes. For example, the adjustable compressor 230 of FIG. 12 may be usedto control the charge density distribution of the plurality of electronbunches along their propagation direction before they enter theundulator 24. Alternatively, the adjustable compressor 230 of FIG. 12may be used to control an average energy of electrons in the pluralityof electron bunches before they enter the undulator 24. In a furtheralternative, the adjustable compressor 230 of FIG. 12 may be used tocontrol both: (i) the control charge density distribution of theplurality of electron bunches along their propagation direction beforethey enter the undulator 24; and (ii) an average energy of the pluralityof electron bunches before they enter the undulator 24. The controllerCT may be operable to switch the adjustable compressor 230 from one modeof operation to another. This may be achieved by altering a phase of theelectromagnetic wave within the resonant cavity 232 relative to theelectron beam E. The controller CT may be provided with an inputmechanism (not shown), which may allow a user to select a mode ofoperation for the adjustable compressor 230.

In order to alter the chirp of the plurality of electron bunches beforethey enter the magnetic compressor 234 (for example in the first mode),the phase of the electromagnetic wave within the resonant cavity 232 maybe such that an electric field within the cavity 232 is substantiallyzero for electrons at a centre of each bunch of the electron beam Epassing through the resonant cavity. For such an arrangement, the changein the chirp of each electron bunch is defined by the amplitude of RFfield oscillations within the resonant cavity 232. Advantageously, sincesuch an arrangement only adjusts the chirp of the electron bunches anddoes not change a mean energy of electrons within the electron bunches,the radio frequency power required to drive the resonant cavity 232 doesnot depend on the average current of the electron beam E. Therefore, thepower required is low and it is possible to use a less efficient,complex and expensive resonant cavity to alter the chirp.

In order to alter an average energy of the electrons in each electronbunch before it enters the undulator 24 (for example in the secondmode), the phase of the electromagnetic wave within the resonant cavity232 may be such that an electric field within the cavity issubstantially at its maximum or minimum value for electrons at a centreof each bunch of the electron beam E passing through the resonant cavity232. With such an arrangement, the resonant cavity 232 tends toaccelerate or decelerate the electron beam E as it propagates throughthe cavity 232, altering the average energy of the electrons within eachbunch. For such an arrangement, the change in the average energy ofelectrons within each bunch is defined by the amplitude of RF fieldoscillations within the resonant cavity 232.

In the first mode of operation, varying the electromagnetic power thatis supplied to the resonant cavity 232 will in turn vary the chargedensity distribution of the electron bunches and therefore the power ofthe radiation beam B output by the undulator 24. In the second mode ofoperation, varying the electromagnetic power that is supplied to theresonant cavity 232 will vary the average energy of the electrons withineach electron bunch and therefore the wavelength (and, to a lesserextent, the power) of the radiation beam B output by the undulator 24.Therefore, since the controller CT varies the electromagnetic power thatis supplied to the resonant cavity 232 in response to the signal Sreceived from the sensor apparatus ST, the arrangement of FIG. 12provides a convenient feedback system for control of the power and/orwavelength of the output radiation beam B. This active feedback systemmay, for example, be used to stabilize a dose of radiation provided bythe radiation beam to a target location on a substrate.

The sensor apparatus ST may be arranged to determine a total powerand/or an intensity distribution of the main radiation beam B output bythe undulator 24. The controller CT may determine, based on the totalpower and/or an intensity distribution of the main radiation beam B andthe spectral response of the optical path between the undulator 24 andthe substrate W, how the dose received at the substrate W will vary asthe electromagnetic power that is supplied to the resonant cavity 232via the amplifier 236 and waveguide 238 is varied.

A dependence of the dose received at a substrate target location on thewavelength and power of the radiation beam B may be determined during acalibration step, prior to exposure of a substrate W. This may beachieved by measuring a dose of radiation received at the substratetable WT for a plurality of different wavelengths and/or total powers ofthe main radiation beam B. During this calibration step (before exposureof a substrate W), the dose may for example be measured using a sensordisposed on the substrate table WT. The dose of energy received by atarget location on a substrate may be an integral with respect to timeof a power of the radiation beam over an exposure time period. Theexposure time period may, for example, be of the order of 1 ms. Theplurality of different wavelengths may be produced by varying theelectromagnetic power that is supplied to the resonant cavity 232 viathe amplifier 236 and waveguide 238 to vary the wavelength. In this way,a calibration map may be determined, which characterizes the dependenceof the dose received by the substrate W on the wavelength and power ofthe radiation beam B. The calibration map may be stored in a memory,which may be accessible by the controller CT. This approach may be usedin connection with other embodiments of the invention which control thepower of the radiation beam B (or other properties of the radiationbeam).

The controller CT may be operable to convert the wavelength, bandwidth,total power and/or an intensity distribution of the main radiation beamB into a dose of radiation that would be received by a target locationon a substrate W during an exposure time period. This conversion may usea calibration map that was determined previously and which may be storedin a memory. Advantageously, this provides a convenient active feed-backsystem for control of the dose of radiation received by a targetlocation on a substrate W, which may, for example, be used to stabilizesaid dose. This approach may be used in connection with otherembodiments of the invention which control the power of the radiationbeam B (or other properties of the radiation beam).

In some embodiments, the resonant cavity 232 is a normally conductingresonant cavity. For example, it may be formed from copper. Normallyconducting resonant cavities such as, for example, copper cavities haverelatively low Q values compared to, for example, the superconductingcavities that are used to accelerate the electron beam E within thelinear accelerator 22. Since the bandwidth of a resonator is inverselyproportional to its Q value, the radio frequency power of such anormally conducting cavity can therefore be adjusted with highbandwidth. Advantageously, this allows for significantly faster changeof the accelerating field gradient within the cavity, as compared to asuperconducting RF cavity. Therefore the use of a normally conductingresonant cavity 232 is particularly beneficial since it allows the powerand/or wavelength of the radiation beam B output by the free electronlaser to be adjusted quickly. This is especially advantageous since itallows a radiation dose supplied by the radiation beam B (for example toa substrate W within one of the lithographic apparatuses LAa-LAn) to becontrolled in real time. It may for example allow the radiation dosesupplied by the radiation beam to be controlled sufficiently quicklythat variations of dose received at target locations are reduced (e.g.control is faster than 1 ms).

Although an arrangement has been discussed wherein the dose of radiationreceived by a target location may be controlled by varying thewavelength of radiation output by a free electron laser, any radiationsource with an adjustable wavelength may alternatively be used for sucha dose control method. Further, although the arrangement of FIG. 12 hasbeen discussed within the context of a lithographic system, it mayalternatively relate to the control of a dose of radiation received by atarget location other than a substrate W within a lithographic apparatusLAa-LS8.

For the arrangement of FIG. 12, wherein the adjustable compressor 230comprises a passive magnetic compressor 234, in order to provide controlover the charge density distribution of the electron beam E along itspropagation direction before it enters the undulator 24 the resonantcavity 232 is disposed ‘up stream’ of the magnetic compressor 234.However if only control over the average energy of the electron beam Ebefore it enters the undulator 24 is desired or required, the resonantcavity 232 may be disposed either ‘up stream’ or ‘down stream’ of themagnetic compressor 234.

FIG. 13 illustrates an alternative arrangement of a free electron lasercomprising an adjustable bunch compressor 260 arranged to continuouslyvary the chirp of the plurality of electron bunches in the electron beamE and an average energy of electron within the bunches. The adjustablebunch compressor 260 comprises a resonant cavity 262 and a magneticcompressor 264. The resonant cavity 262 is disposed ‘up stream’ of themagnetic compressor 264, i.e. the electron beam E passes first throughthe resonant cavity 262 and then through the magnetic compressor 264.

The magnetic compressor 264 is arranged to compress the electron buncheswith the electron beam E along a direction of propagation of theelectron beam, the compression being dependent on the chirp of theelectron bunches as they enter the magnetic compressor 264.

The resonant cavity 262 is separate from the linear accelerator 22. Theresonant cavity 262 is provided with an amplifier 266 andelectromagnetic energy is communicated from the amplifier 266 to theresonant cavity 262 via a waveguide 268.

The resonant cavity 262 is arranged to operate at a frequency that is,in general, different from that of the electron beam E such that a chirpof the electron bunches and/or an average energy of the electron bunchesvaries with time. This may be achieved by using different low powerradio frequency power sources 225, 265 to supply electromagnetic energyto the linear accelerator 22 and the resonant cavity 262 respectively.

When the frequency of the resonant cavity 262 is different to that ofthe electron beam E, the resonant cavity 262 will continuously vary thechirp the electron bunches and the mean energy of electrons within theelectron bunches. The rate of change of the chirp and mean energy isdependent upon difference between the frequencies of the linearaccelerator 22 and the resonant cavity 262. Altering the mean energy ofelectrons within the electron bunches will alter the wavelength of theradiation beam (as the mean energy increases, the wavelength of theradiation beam decreases). Therefore the arrangement of FIG. 13 providesa mechanism for increasing the effective bandwidth of the radiationoutput by a free electron laser.

In some embodiments, the resonant cavity 262 is a normally conductingresonant cavity. For example, it may be formed from copper. Normallyconducting resonant cavities such as, for example, copper cavities haverelatively low Q values compared to, for example, the superconductingcavities that are used to accelerate the electron beam E within thelinear accelerator 22. Since the bandwidth of a resonator is inverselyproportional to its Q value, the radio frequency power of such anormally conducting cavity can therefore be adjusted with highbandwidth. Advantageously, this allows for significantly faster changeof the accelerating field gradient within the cavity, as compared to asuperconducting RF cavity. Therefore the use of a normally conductingresonant cavity 262 is particularly beneficial since it allows the powerof the radiation beam B output by the free electron laser to be adjustedquickly. It may for example allow the radiation dose supplied by theradiation beam to be controlled sufficiently quickly that variations ofdose received at target locations are reduced (e.g. control is fasterthan 1 ms).

The free electron laser of FIG. 13 further comprises a controller CT.The controller CT is operable to receive an input signal 51 from asensor apparatus ST. In response to the signal 51 the controller CT isoperable to vary one or more parameters of the resonant cavity 262. Forexample, the controller CT may be operable to vary the electromagneticpower that is supplied to the resonant cavity 262 via the amplifier 266and waveguide 268. Alternatively or additionally, the controller CT maybe operable to vary the frequency of the electromagnetic standing wavewithin the radio frequency cavity 262. This may be achieved by adjustingboth: (a) a frequency of the electromagnetic radiation supplied to theresonant cavity 262 by the low power source 265; and (b) a geometry ofthe resonant cavity 262 to maintain a resonance condition. The geometryof the resonant cavity 262 may be altered using, for example, one ormore piezoelectric stretchers and/or compressors to match a resonantfrequency of the resonant cavity 262 to the frequency of the low powerradio frequency source 265.

The sensor apparatus ST may be arranged to output a value indicative ofa power of the radiation beam B. Advantageously, this provides aconvenient feedback-based control loop for control of the power of theoutput radiation beam B, which may, for example, be used to stabilizesaid power. The controller CT may use the power measured by the sensorapparatus ST, together with the calibration described above, tocalculate a dose of radiation delivered to a target location on asubstrate by the radiation beam B, and to adjust the power of theradiation beam accordingly. The dose of energy received by the targetlocation may be an integral with respect to time of a power of theradiation beam over an exposure time period. The exposure time periodmay, for example, be of the order of 1 ms.

The described embodiments of a free electron laser comprise anadjustable bunch compressor 230, 260 disposed downstream of the linearaccelerator 22 and upstream of the undulator 24. That is, the electronbeam E passes through the linear accelerator 22, the adjustable bunchcompressor 230, 260 and the undulator 24 in that order. However, inalternative embodiments the free electron may comprise an adjustablebunch compressor which is disposed upstream of the linear accelerator22. As with the above embodiments, this adjustable bunch compressor maybe operable to control at least one of: (i) a charge densitydistribution of each of the plurality of electron bunches along adirection of propagation of the electron beam before it enters theundulator; or (ii) an average energy of each of the plurality ofelectron bunches before it enters the undulator. For example, theadjustable bunch compressor may comprise a beam buncher within theinjector 21, which may comprise a resonant cavity. For such embodiments,the electrons within the electron bunches are not relativistic.Therefore an energy chirp imposed by the beam buncher may cause asignificant difference of velocities of electrons at a head and a tailof each bunch. Therefore, for such embodiments the adjustable compressormay not comprise a magnetic compressor.

FIG. 14 shows schematically an undulator 24 according to an embodimentof the invention. The undulator comprises three modules 300. Eachundulator module 300 comprises a periodic magnet structure which guidesan electron beam E along a periodic path such that the electrons radiateelectromagnetic radiation in the direction of a central axis of theirperiodic path, thereby forming an EUV radiation beam B (which may beconsidered to be a laser radiation beam). Gaps 302 are provided betweenthe undulator modules 300. Dynamic phase shifters 304 are located in thegaps. The term “dynamic phase shifter” may be interpreted as meaning asphase shifter which can be controlled to apply a phase shift or notapply a phase shift, and/or controlled to apply phase shifts ofdifferent sizes and/or magnitudes. The dynamic phase shifters 304 arecontrolled by a controller CT. Although three undulator modules 300 areshown in FIG. 14, the undulator 24 may comprise more undulator modules(or less undulator modules). Similarly, although two dynamic phaseshifters 304 are shown in FIG. 14, more than two dynamic phase shiftersmay be provided or a single dynamic phase shifter may be provided.

During operation of the free electron laser the efficiency with whichthe electron beam power is converted to laser radiation beam power isaffected by the relative phase between the oscillating motion ofelectrons in the periodic magnetic field of the undulator modules 300and the phase of the electromagnetic wave of the radiation beam (i.e.the phase of radiation already produced upstream in the undulator byoscillating motion of the electrons).

The gaps 302 between undulator modules 300 each introduce a phase shiftbetween the electron transverse velocity and the phase of theelectromagnetic field of the radiation beam B. When these are in phase(i.e. the phase difference is close 2*π*N, where N is an integer),energy is transferred from the electrons to the radiation beam B (thisis the amplification process of the free electron laser). When the phasedifference is close to (2*N+1)*π, the phase difference causes theelectrons to gain energy from the radiation beam, thus reversing theamplification process of the free electron laser. The phase shift φintroduced by a gap between two undulator modules is governed by:

$\begin{matrix}{\phi = {\frac{L_{g}}{2\gamma^{2}\lambda_{r}} = \frac{L_{g}}{\lambda_{u}\left( {1 + \frac{K^{2}}{A}} \right)}}} & (4)\end{matrix}$

where L_(g) is the length of the gap 302 (the distance from the end of afirst undulator module to the beginning of the next undulator module), γis the Lorentz factor of the electrons, λ_(r) is the wavelength ofradiation, λ_(U) is the period of the undulator, K is the undulatorparameter and A is dependent upon the geometry of the undulator and theresulting radiation beam polarization (as explained above in connectionwith Eq. (1).

As can be seen from Eq. (4) in normal circumstances the phase shift φintroduced by each gap 302 is fixed. However, the phase shift may bemodified using a dynamic phase shifter 304. The controller CT maycontrol the dynamic phase shifter to control the phase shift and therebycontrol the efficiency with which the electron beam is converted tophotons. Thus, the controller CT may use the dynamic phase shifter tocontrol the power of the EUV radiation beam B emitted from the undulator24.

FIG. 15 shows schematically an example of a dynamic phase shifter 304.The dynamic phase shifter 304 comprises three pairs of electromagnetics306-308, an electromagnet of each pair being located on either side ofthe trajectory of the electron beam E. The first pair of electromagnets306 a,b comprises a first magnet 306 a formed from ferrite material andloops of wire 310 arranged such that a south pole which faces theelectron beam trajectory is generated when current is supplied to thewire. The second electromagnet 306 b of the pair comprises ferritematerial with loops of wire 310 arranged such that when current passesthrough the wire a north pole which faces the electron beam trajectoryis generated. Thus, when current flows through the wires a magneticfield extends across the electron beam trajectory. When current does notflow through the wires the magnetic field is not present.

The second electromagnet pair 307 a,b similarly comprises two pieces offerrite material around which loops of wire 310 are provided. However,in this case the north and south poles generated by current passingthrough the wires 310 are on opposite sides of the electron beamtrajectory (relative to the first pair of electromagnets 306 a,b). Inaddition, the second electromagnet pair 307 a,b is configured togenerate a magnetic field which is double the size of the magnetic fieldgenerated by the first electromagnet pair. Thus, when current flowsthrough the wires a magnetic field extends across the electron beamtrajectory which is opposite in sign and twice as large as the magneticfield generated by the first electromagnet pair 306 a,b.

The third pair of electromagnets 308 a,b has the same configuration asthe first pair of electromagnets 306 a,b. Thus, when current flowsthrough the wires this pair of electromagnets 308 a,b provides amagnetic field which extends across the electron beam trajectory withthe same sign and magnitude as the magnetic field generated by the firstpair of electromagnets 306 a,b.

In use, when no current is flowing through the wires of theelectromagnets 306-308 the electron beam E travels without itstrajectory being modified (i.e. along the trajectory indicated by thedashed line E1). When current flows through the wires the electromagnets306-308 introduce bends into the trajectory of the electron beam E suchthat the electron beam follows the longer path indicated by the solidline E2 in FIG. 15.

The first pair of electromagnets 306 a,b bends the trajectory of theelectron beam in a first direction (upwards in FIG. 15). The second pairof electromagnets 307 a,b applies a bend in the opposite direction withtwice the magnitude (the electron beam bends downwards in FIG. 15).Finally, the third pair of electromagnets 308 a,b applies a further bendto the electron beam (upwards in FIG. 15). This further bend correspondswith the bend applied by the first pair of electromagnets 306 a,b and asa result returns the electron beam E to its initial trajectory. Thetrajectory of the electron beam E2 when the electromagnets are activemay be referred to as a chicane.

As may be seen from FIG. 15, the trajectory of the electron beam E onleaving the dynamic phase shifter 304 is the same irrespective ofwhether or not current is flowing through wires 310 of theelectromagnets 306-308 (i.e. irrespective of whether or not theelectromagnets are active). However, the length of the trajectory E2traveled by the electron beam when the electromagnets are active isgreater than the length of the trajectory E1 traveled by the electronbeam when the electromagnets are not active. Consequently, activatingthe electromagnets 306-308 introduces a phase shift into the electronbeam E which is not present when the electromagnets are not active.Since the phase shift affects the conversion efficiency of the electronbeam to EUV radiation, the electromagnets 306-308 can be used to changethe power of the EUV radiation beam emitted by the undulator 24 of thefree electron laser FEL.

The phase difference introduced into the electron beam E by the dynamicphase shifter 304 may be calculated as follows:

$\begin{matrix}{{\Delta\;\phi} = {\frac{2\pi\;\Delta\; l}{\lambda_{r}}\left( \frac{e}{m\mspace{11mu} c\;\gamma} \right)^{2}{\left. \left( {l_{m}B_{0}} \right)^{2} \right.\sim 2.158}*10^{9}\frac{\Delta\;{l\lbrack m\rbrack}}{\lambda_{r}\lbrack{nm}\rbrack}\left( \frac{l_{m}{B_{0}\lbrack{Tmm}\rbrack}}{\gamma} \right)^{2}}} & (5)\end{matrix}$where Δl is the distance between centers of the first pair ofelectromagnets 306 a,b and the second pair of electromagnets 307 a,balong the electron trajectory, m is the electron mass, c is the speed oflight, l_(m) is the length of the first electromagnet pair along theelectron trajectory, and B₀ is the strength of the magnetic fieldgenerated by the first electromagnet pair (and by the thirdelectromagnet pair 308).

In an embodiment, it may be desired to apply a phase shift of π usingthe dynamic phase shifter 304. In a free electron laser generating EUVradiation the following parameters may apply: λ_(r)=13.5 nm, γ˜1500,Δl=0.5 m and l_(m)=0.1 m. Where these parameters apply the magnitude ofthe magnetic field B₀ needed in order to introduce a phase shift of π isaround 0.01 T. This is a relatively small magnetic field and may begenerated using an appropriate ferrite material such as MnZn or NiZn.

The size of the bend of electron trajectory (which may be referred to asa kick angle) applied by a pair of electromagnets 306-308 may beestimated as follows:

$\begin{matrix}{{\alpha\lbrack{Rad}\rbrack} = {0.586*\left( \frac{l_{m}{B_{0}\lbrack{Tmm}\rbrack}}{\gamma} \right)}} & (6)\end{matrix}$

When the above example parameters are used this will increase the pathlength of the electron beam trajectory by around 0.03 mm.

The electromagnets 306-308 of the dynamic phase shifter 304 may all beactivated by the same wire 310. That is, current from a single sourcepasses through each of the electromagnets 306-308. An advantage of thisarrangement is that it ensures that the electromagnets 306-308 are allactivated together and switched off together. This avoids thepossibility that one electromagnet is switched on without the others,which would cause deviation of the beam trajectory output from thedynamic phase shifter 304. The deviated beam would be likely to beincident upon a component of the free electron laser and cause damage tothat component.

The electron beam E may be surrounded by a protective tube 320. Theprotective tube 320 acts to protect components outside of the tube fromwake-field disturbances and from electrons which are lost from theelectron beam E. The protective tube 320 also seals the electron beam Efrom an external environment in order to allow a vacuum to beestablished. The protective tube 320 may be made from a conductingmaterial such as copper or aluminum. The protective tube 320 may besupported by a supporting tube 321 which may be formed from a dielectricmaterial.

As shown in FIG. 16, the protective tube 320 may be provided withopenings 322 which run parallel to the electron beam trajectory. Theopenings 322 are positioned to allow the magnetic fields generated bythe electromagnets 306-308 to pass through the protective tube such thatthey can act to modify the trajectory of the electron beam E. Theopenings may be sealed (e.g. using dielectric material) in order toprevent leakage of vacuum from within the protective tube 320. Theopenings 322 may have tapered ends in order to minimize wake-fieldinduced heating and electron bunch degradation (which could occur, forexample, if the openings had squared-off ends).

In an alternative arrangement, instead of providing holes in theprotective tube 320 the thickness of the tube may be made thinner thanthe skin layer thickness which is expected for a frequency of operationof the electromagnets 306-308. For example, if the electromagnetsgenerate an oscillating magnetic field at a frequency of around 100 kHz,then the skin depth for copper or aluminum will be a few hundredmicrons. The frequency of wake-field disturbances associated withelectron bunches of the electron beam E is of the order of GHz, and thusa field penetration depth of a few microns may be expected. Therefore, aconducting protective tube 320 with a wall thickness which is greaterthan a few microns but less than a few hundred microns will protect theelectromagnets 306-308 from wake-field disturbances whilst at the sametime allowing the electromagnetic field generated by the electromagnets306-308 to modify the trajectory of the electron beam. In embodiments inwhich the conductive wall is thin (e.g. hundreds of microns or less),the supporting tube 321 may provide structural support for theconductive wall. In general, the conductive wall thickness may begreater than 10 microns. In general, the conductive wall thickness maybe less than 1 mm.

The above described alternative arrangements may be combined. Forexample, openings 322 in a metal protective tube 320 that are filledwith dielectric material may in addition be provided with a thin layerof conducting material (e.g. on an inner surface). This is beneficialsince it prevents charging of the dielectric material and thus preventsdamage occurring due to subsequent electrical discharges.

An alternative embodiment of a dynamic phase shifter 304 is shownschematically in FIG. 17. The alternative embodiment comprises threetransverse kick cavities 330-332. The transverse kick cavities aregenerally cylindrical cavities having a central axis which correspondswith the electron beam trajectory E and which generate electromagneticfields that cause the electron beam to deviate from a straight-line pathE1 (i.e. to follow a chicane path E2 as shown schematically). Thetransverse kick cavities 330-332 are activated by supplying RF power tothem. Thus, the transverse kick cavities 330-332 may be activated andswitched off by supplying and removing RF power. Alternatively, RF powermay be supplied continuously to the transverse kick cavities, and theshapes of the cavities may be adjusted to move the cavities into and outof resonance with the RF power. The kicking field is at its nominal sizewhen the cavity is in resonance and is approximately zero when thecavity is out of resonance. The shapes of the cavities may be adjustedusing tuning elements (not shown) provided in the cavities.

The effect of the transverse kick cavities 330-332 is identical to theeffect of the electromagnets 306-308. The transverse kick cavities330-332 may be used to selectively introduce a phase shift to theelectron beam E. As explained above, the phase shift will in turn affectthe conversion efficiency of the undulator and therefore affect thepower of EUV radiation output from the free electron laser.

In an embodiment, the separation between adjacent undulator modules 300may be selected to provide an electron beam phase change which is aninteger multiple of 2π. Where this is done, the relative phase betweenthe oscillating motion of electrons in periodic magnetic field in theundulator modules 300 and the electromagnetic field of the radiationbeam B does not change between undulator modules when the dynamic phaseshifter 304 is not active. In an embodiment, the dynamic phase shifter304 may configured to apply a phase shift of approximately π. Thus,modulating operation of the dynamic phase shifter will apply a π phaseshift to the electron beam.

As explained further above, it may be desirable to control the power ofthe EUV radiation beam generated by the free electron laser FEL with acontrol frequency of around 10 kHz or higher (e.g. around 100 kHz ormore). This will allow control of the dose of EUV radiation received bya target location on a substrate (for example if the target location isexposed for 1 ms). The dynamic phase shifter 304 may thus be configuredto operate at a frequency of 10 kHz or higher. For an embodiment whichuses electromagnets a ferrite material which is capable of operating atsuch frequencies may be used. MnZn or NiZn are examples of ferritematerial which is capable of operating at these frequencies. In general,fast ferrite material may be used. In an embodiment, one or more of theelectromagnets may be an air-coil (i.e. a loops of wire without a coreof ferrite material). For embodiments which use transverse kick cavitiesan RF power supply with a control frequency of 10 kHz or higher may beused. Additionally or alternatively a cavity tuning element with acontrol frequency of 10 kHz or higher may be used.

An advantage of using a dynamic phase shifter 304 to modify the power ofEUV radiation emitted by the free electron laser FEL is that it does notaffect properties of electron bunches before they arrive at theundulator 24. Thus, referring to FIG. 3, the accelerator 22 used toaccelerate electron bunches of the electron beam E may be unaffected. Ifthe free electron laser FEL uses recirculation of the electrons throughthe accelerator 22 to decelerate electrons after generation ofelectromagnetic radiation by the undulator 24, then the electrons whichpass into the accelerator for deceleration will have been affected bythe dynamic phase shifter 304. However, a change of energy of theelectrons caused by the dynamic phase shifter 304 will typically bearound 0.1%, and thus has only a very minor influence upon theaccelerator 22.

In an embodiment, a pair of dynamic phase shifters 304 may be provided(e.g. as shown in FIG. 14), each dynamic phase shifter being provided ina different gap 302. Where this is done the first dynamic phase shiftermay be arranged to apply a first phase shift and the second dynamicphase shifter arranged to apply a second phase shift. The first andsecond phase shifts may for example have different magnitudes. The firstand second phase shifts may for example have the same magnitudes butopposite signs.

In one control scheme the phase shifts applied by the dynamic phaseshifters 304 may be equal in magnitude but opposite in sign. If a phaseshift is already present in the electron beam E before it reaches thedynamic phase shifters then one of the dynamic phase shifters 304 mayact to increase the size of that phase shift and the other may act todecrease the size of the phase shift. The combined effect of the twodynamic phase shifters 304 on the electron beam E will be no resultantchange of phase. Thus, the conversion efficiency of the undulator 24when the dynamic phase shifters 304 are active will be the same as theconversion efficiency when the dynamic phase shifters 304 are notactive.

Alternatively, a control scheme may be implemented such that only one ofthe two dynamic phase-shifters is active at any given time. Operation ofeither dynamic phase shifter will modify radiation amplification in theundulator by the same amount. As a result, the output radiation beamwill have the same power irrespective of which dynamic phase shifter isactive.

Alternatively, a control scheme may be implemented such that the twodynamic phase shifters are adjusted together, the adjustments havingmagnitudes and signs such that the power of the output radiation beamdoes not change. The adjustments may be pre-determined magnitudes andsigns and/or may be calibrated and monitored magnitudes and signs.

In all three above described control schemes the power of the EUVradiation beam will remain the same. However, because the phase ofelectrons when they are travelling between the two dynamic phaseshifters 304 is modified, this will alter generation of the radiationbeam E in the undulator 300 which is between the dynamic phase shifters.The radiation beam E will be generated with a modified bandwidth and/orspatial power distribution of the radiation in the undulator 300 whichis between the dynamic phase shifters. Radiation beam generation mayalso be modified in undulators downstream of dynamic phase shifters.Therefore, when the dynamic phase shifters 304 are active the radiationbeam E is generated with the same power but a different bandwidth and/orpower distribution (compared with the radiation beam generated when thedynamic phase shifters are not active). As explained elsewhere in thisdocument, the collective transmission of mirrors of the lithographicsystem is wavelength dependent. The mirrors also have finitespatial/angular acceptance. Therefore, changing the radiation beambandwidth and/or power distribution using the dynamic phase shifters 304may be used to control the dose of radiation delivered to a substrate bya lithographic apparatus. The effect of bandwidth and/or spatial powerdistribution changes caused by dynamic phase shifters 304 on the powerof radiation delivered by a lithographic apparatus projection system PSmay be calibrated and the results of the calibration used by thecontroller CT when controlling the dynamic phase shifters 304.

The above approach can be generalized to use more than two dynamic phaseshifters. The effect of different combinations of phase shifters on theradiation beam power delivered by a lithographic apparatus projectionsystem PS may be measured and then subsequently used by the controllerCT to control the radiation beam bandwidth and/or spatial powerdistribution. For example, ten dynamic phase shifters may be provided,and different combinations of dynamic phase shifters may be activatedand switched off by the controller CT in order to achieve differentvariations of the power of the radiation beam delivered by theprojection system (e.g. without significantly changing the power of theradiation beam as output from the free electron laser).

Embodiments have been described in which control of the dynamic phaseshifters 304 comprises switching between the dynamic phase shiftersbeing active and not being active (e.g. modulated between being on andoff). The size of the phase shift applied by a dynamic phase shifterwhen it is active may also be controlled (e.g. by the controller CT).This may be achieved for example by adjusting the size of the currentsupplied to the electromagnets 306-308, thereby adjusting the size ofthe phase shift that is applied by the dynamic phase shifter.

FIGS. 18-27 depict various configurations of sensor apparatus which maycomprise the sensor apparatus ST referred to above in connection withvarious embodiments of the invention.

Referring first to FIG. 18, a first embodiment of a sensor apparatus 400for determining a value indicative of a power of an EUV radiation beam Bis shown. The sensor apparatus 400 comprises a sensor 410 and an opticalelement 420 for receiving a main radiation beam B_(m). The sensor 410may comprise an array of sensing elements such as, for example, chargedcoupled devices (CCDs) and/or photodiodes. The optical element 420 is amirror, which may be a grazing incidence mirror. The EUV radiation beamB may, for example, be the primary radiation beam B produced by the freeelectron laser FEL or one of the secondary radiation beams B_(a)-B_(h)produced by the beam splitting apparatus 20.

The sensor 410 is disposed in a sensing environment 415 and the opticalelement 420 is disposed in a main beam environment 425. In general,conditions within the sensing environment 415 may differ from those inthe main beam environment 425. For example, the radiation beam B maycomprise EUV radiation and therefore the main beam environment 425 maybe held at vacuum conditions. In this embodiment, a wall 430 separatesthe sensing environment 415 from the main beam environment 425. Atransparent membrane or window 431 is provided in the wall 430.

Referring to FIG. 19, a reflective surface 421 of the optical element420 is generally smooth but is provided with a plurality of marks 422distributed over the reflective surface. In the present embodiment, eachof the plurality of marks 422 is of the form of a generallyhemi-spherical recess in the reflective surface 421. The plurality ofmarks 422 may for example be etched into the reflective surface usingany suitable process such as, for example, ion milling. The plurality ofmarks 422 form a first region of the optical element 420 that isarranged to receive a first portion of the radiation beam B. Theremaining substantially smooth portion of the reflective surface forms asecond region of the optical element 420 that is arranged to receive asecond portion of the radiation beam B. Since these first and secondregions of the optical element form spatially distinct regions of thereflective surface 421, the first and second portions of the radiationbeam B correspond to different regions of a spatial intensitydistribution of the radiation beam B.

The first portion of the radiation beam is scattered by the plurality ofmarks 422 to form a first branch radiation beam B₁. This scattering issuch that the first branch radiation beam B₁ is directed through thewindow 431 to the sensing environment 415. The second portion of theradiation beam B is reflected by the second region to form a secondbranch radiation beam B₂. The second branch radiation beam B₂ remainswithin the main beam environment 425 and may, for example, be directedto one or more of the lithographic apparatuses LA_(a)-LA_(n).

A screen 411 of fluorescent material is provided over the sensor 410.Within the sensing environment 415, the first branch radiation beam B₁is directed to the sensor 410 by an optical element 440. Optical element440 produces an image of the plurality of marks 422 on the screen 411.This image may be focused or de-focused. The first branch radiation beamB₁ is absorbed by the screen 411 of fluorescent material, which emitsradiation with a longer wavelength. This emitted radiation is detectedby the sensor 410. The use of such a fluorescent screen significantlysimplifies power measurements for radiation beams with relatively shortpulses. For example, a free electron laser may produce sub-picosecondpulses (a typical pulse may be of the order of 100 fs). Such shortpulses may be too short to be resolved by known sensing elements, suchas fast photo diodes. However, fluorescence typically occurs overnanosecond time scales, even if the fluorescent material is excited byfemtosecond pulses. Therefore the radiation emitted by the fluorescentscreen can be resolved using known sensing elements. Suitablefluorescent materials include zinc oxide (ZnO), which is a semiconductorgrade material that is routinely produced in single crystal disks up to3 inches in diameter, or yttrium aluminium garnet (YAG) doped with arare earth element such as, for example cerium (YAG:Ce).

The sensor 410 is connected to a controller CT by a cable 413. Thesensor 410 is operable to send a signal to the controller CT which isindicative of the power determined by the sensor 410.

Advantageously, this sensor apparatus 400 allows a power of a firstportion of the radiation beam B to be determined without requiringsensors to be placed in the path of the radiation beam B. Therefore, theinvention enables the measurement of the power of radiation beams withvery high powers and intensities, which would otherwise place too high athermal load on sensors placed directly in their path. For example, itenables measurement of the power of a primary radiation beam B producedby the free electron laser that provides radiation to a plurality oflithographic apparatuses LA_(a)-LA_(n). Such a radiation beam may have apower of the order of tens of kilowatts and a relatively small etendue.

In addition, since sensors need not be placed in the path of theradiation beam B, this embodiment of the invention provides anarrangement wherein there are no limits on the dimensions of theplurality of marks 422. In particular, this allows the marks 422 to besufficiently small that the part of the intensity distribution that isused for the power measurement (i.e. the part that contributes to thefirst branch radiation beam) is significantly smaller than would be thecase if one or more sensors were placed in the path of the radiationbeam.

Referring to FIGS. 19 and 20, the plurality of marks 422 are distributedover the reflective surface 421. In this embodiment the plurality ofmarks 422 form a rectangular lattice over the reflective surface 421,with a spacing between adjacent marks of l in a first direction and aspacing between adjacent marks of h in a second, perpendiculardirection. Alternative embodiments may use other distributions of marks422 over the reflective surface. A dimension d of each mark 422 issignificantly smaller than the spacing between adjacent marks l, h.Advantageously, this ensures that the first portion of the radiationbeam B that is used for power measurement is relatively small.

The dimension d of each mark 422 may be relatively small. For example,the dimension d of each mark 422 may be smaller than around 100 μm.

In a near field, close to the optical element, the power distribution ofthe second branch radiation beam B₂ will be similar to that of theradiation beam B except for a plurality of gaps, each corresponding to adifferent one of the marks 422, where the power distribution issubstantially zero. The second branch radiation beam B₂ may, forexample, be directed to one or more of the lithographic apparatusesLA_(a)-LA_(n), which may be disposed in a far field of the opticalelement 420. For such embodiments, the dimension d of each mark 422 ispreferably sufficiently small that in the far field the plurality ofgaps in the power distribution corresponding to the marks 422 have beensmoothed out by diffraction due to the divergence of the second branchradiation beam B₂.

Further, the dimension d of each mark 422 is preferably sufficientlysmall that when the mirror is illuminated by the radiation beam B mirrorthermal expansion distortion and disturbance of the shape of thereflective surface 421 in the proximity of the marks 422 is negligible.Advantageously, this ensures that focusing effects due to variation ofthermal expansion are negligible or can be corrected for.

Further, the dimension d of each mark 422 is preferably sufficientlysmall that power emitted or scattered by a single mark is relativelysmall (e.g. less than 1%). Advantageously, this ensures that no, orrelatively little, attenuation is required to before it can be measureddirectly.

The first branch radiation beam B₁ comprises a part corresponding toeach of the plurality of marks 422. The plurality of marks 422 and theoptical element 420 may be arranged such that each such part is directedto a different spatial part of the fluorescent screen 411. Thefluorescent screen 411 may be operable to emit a separate beam ofradiation for each such part. In such embodiments, the sensor 410 isoperable to determine the power of each such beam of radiation emittedby the fluorescent screen 411. Therefore, referring to FIG. 21, thesensor 410 may be operable to determine the power of the radiation beamB at a number of discrete points across its beam profile. As such thesensor may be operable to output a signal that is indicative of adiscretely sampled intensity distribution 414 of the radiation beam.

The controller CT may be operable to determine a power distribution ofthe radiation beam from the discretely sampled intensity distribution414 by interpolation. For example, an expected beam profile shape may beassumed and a number of parameters of the profile shape may be fit tothe data output by the sensor 410. This may use, for example, a leastsquares algorithm.

The controller CT may be operable to use the determined power orintensity distribution to control an aspect of the radiation beam B. Forexample, the controller CT may be connected to the free electron laserFEL to control a parameter of the source free electron laser based onthe determined power or intensity distribution. For example, thecontroller may be arranged to adjust the direction and/or position theradiation beam B, and/or to adjust the power intensity or the intensitydistribution of the radiation beam B.

Referring to FIG. 22, in an alternative embodiment the plurality ofmarks 422 may be of the form of generally hemi-spherical protrusions onthe reflective surface 421. In other alternative embodiments, the marks422 may comprise differently shaped recesses or protrusions.

In the embodiment shown in FIG. 18, the wall 430 with the transparentmembrane or window 431 is provided to separate the sensing environment415 from the main beam environment 425. An alternative embodiment of asensor apparatus 400 a for determining a value indicative of a power ofa radiation beam is shown in FIG. 23. Features of sensor apparatus 400 athat directly correspond to sensor apparatus 400 have the same labels.Only the differences between sensor apparatus 400 a and sensor apparatus400 will be described in detail here. In this alternative embodiment allEUV optics, including optical element 440 that is arranged to direct thefirst branch radiation beam B₁ to the fluorescent screen 411 aredisposed in a main beam environment 425 a, whereas the sensor 410 isdisposed in a sensing environment 415 a. A wall 430 a is provided toseparate the sensing environment 415 a from the main beam environment425 a, which may in general be held under different conditions. In thisalternative embodiment of a sensor apparatus 400 a, the fluorescentscreen 411 acts as a window in the wall 430 a, separating the sensingenvironment 415 a from the main beam environment 425 a.

Referring to FIG. 24, a further alternative embodiment of a sensorapparatus 400 b for determining a value indicative of a power of aradiation beam is shown. Features of sensor apparatus 400 b thatdirectly correspond to sensor apparatus 400 have the same labels. Onlythe differences between sensor apparatus 400 b and sensor apparatus 400will be described in detail here. In this alternative embodiment, areflective surface 421 of the optical element 420 is generally smoothbut is provided with a plurality of fluorescent marks 422 a distributedover the reflective surface. In the present embodiment, each of theplurality of fluorescent marks 422 a is of the form of a generallyhemi-spherical protrusion formed from a fluorescent material.

The first portion of the radiation beam is absorbed by the plurality offluorescent marks 422 a, which emits radiation with a longer wavelengthto form a first branch radiation beam B₁. The fluorescent marks 422 aare arranged such that the first branch radiation beam B₁ is directedthrough window 431 to sensing environment 415. The use of such afluorescent screen simplifies power measurements for radiation beamswith relatively short pulses. Suitable fluorescent materials includezinc oxide (ZnO), which is a semiconductor grade material that isroutinely produced in single crystal disks up to 3 inches in diameter,or yttrium aluminium garnet (YAG) doped with a rare earth element suchas, for example cerium (YAG:Ce).

As with the previous embodiments, the second portion of the radiationbeam B is reflected by the second region to form a second branchradiation beam B₂ that remains within the main beam environment 425 andmay, for example, be directed to one or more of the lithographicapparatuses LAa-LAn.

The first branch radiation beam B₁, which comprises radiation with awavelength longer than EUV radiation, is directed to the sensor 410 viadedicated optics. No fluorescent screen is provided since the firstradiation beam B₁ already comprises radiation of a longer wavelength andpulses that are on the scale of the fluorescence process (typicallynanosecond time scales rather than femtosecond time scales). In thisembodiment the dedicated optics comprises a reflective optical element441 and a focusing optical element 442. Other combinations of opticalelements may alternatively be used as required. The dedicated optics441, 442 and the sensor 410 are disposed in a sensing environment 415. Arelative advantage of the sensor apparatus 400 b over, for example,sensor apparatus 400 is that the first branch radiation beam does notcomprise EUV radiation and therefore a cheaper and simpler opticalarrangement and sensing environment 415 may be used for the first branchradiation beam B₁. For example, lenses may be used rather than expensiveEUV mirrors and the sensing environment 415 may, for example, compriseair at atmospheric pressure.

A relative advantage of the sensor apparatus 400 over sensor apparatus400 b is that the fluorescent material is provided on the reflectivesurface 421 of the optical element (fluorescent marks 422 a) rather thanin the sensing material (fluorescent screen 411). Therefore thefluorescent material is not exposed to such high power EUV radiation insensor apparatus 400 and may therefore be expected to have a longerlifetime. Further, in sensor apparatus 400 b, the fluorescent marks 422a may experience different temperature changes depending on whichportion of the radiation beam B profile is incident thereon. Since thefluorescence process may be temperature dependent, this may make it moredifficult to accurately map the distribution determined by the sensor410 to the intensity profile of the radiation beam B.

Referring to FIG. 25, a further embodiment of a sensor apparatus 470according to the present invention for determining a value indicative ofa power of a radiation beam is shown. The sensor apparatus 470 comprisesa sensor 471 and an optical element 472 for receiving a radiation beamB. The sensor 471 may be substantially similar to sensor 410 describedabove. In particular, sensor 471 may comprise an array of sensingelements such as, for example, charged coupled devices (CCDs) and/orphotodiodes.

The optical element 472 is a mirror, which may be a grazing incidencemirror. As with previous embodiments, the radiation beam B may, forexample, be the primary radiation beam B produced by the free electronlaser FEL or one of the secondary radiation beams B_(a)-B_(h) producedby the beam splitting apparatus 20.

Optical element 472 comprises a reflective surface 473. A plurality ofregularly spaces grooves extend across the reflective surface 473. Thegrooves may be formed by any suitable process such as, for example,etching or stamping.

The optical element 472 may form part of the beam splitting apparatus 20of FIG. 1 and may be disposed at a distance of the order of tens orhundreds of meters from the output of the undulator 24 for thermalreasons. Similarly, the optical element 472 may be a grazing incidencemirror with a relatively small grazing incidence angle such as, forexample, or the order of 1 to 4 degrees.

The optical element 472 can be formed from silicon by, for example,etching along crystal planes of the silicon. Referring to FIG. 26, anexample of a reflective surface 473 of the optical element 472 is shownfor an embodiment wherein the optical element 472 is formed fromsilicon. In this illustrated example, the top faces 475 are formed fromthe <100> crystallographic plane and the faces 476 a, 476 b forming thegrooves can be formed from the <111> and <−111> planes. With such anarrangement, the angle at the bottom of the grooves is 70.529 degrees.The grooves run along the <01-1> direction. The direction of theincoming radiation beam B is disposed at a small (grazing incidence)angle to the <01-1> direction. Such a grating would form three branchradiation beams, which may be considered to be the 0th and ±1st orders.The ratios of intensities of the branch radiation beams are dependent onthe ratios of the areas of the faces from which they are reflected (forexample top faces 475 or the faces 476 a, 476 b forming the grooves) andon the angle of incidence of the incoming radiation beam B.

The optical element 472 may be provided with a coating of a morereflective material (for EUV radiation). For example, the opticalelement may be provided with a coating of ruthenium (Ru). This may, forexample, have a thickness of around 50 nm.

An advantage of using silicon for the optical element 472 is that itsthermal expansion during operation may be limited by operating at about123 K. At this temperature the heat conductivity of silicon is of theorder of 400 b W/m/K or more, which is a factor of 4 better than itsheat conductivity at room temperature and around 50% better than copper(Cu). Therefore, even a relatively large heat load can be drained, whilekeeping the temperature in the range where expansion is low and theoptical element 472 will keep its designed structural dimensions.

The grooves divide the reflective surface 473 into a plurality of groupsof surface elements. Each group of surface elements comprises aplurality of substantially parallel surface elements. For example, thetop faces 475 of FIG. 26 form a first group of surface elements, faces476 a forming one side of each ridge form a second group of surfaceelements and faces 476 b forming an opposite side of each ridge form athird group of surface elements. Although only a small section of theoptical element is shown in FIG. 26, each group may comprise of theorder of 1000 surface elements. Such an arrangement acts as a reflectivegrating. Each group of surface elements forms a different region of theoptical element 472 that is arranged to receive a different portion ofthe radiation beam B. Since these different regions of the opticalelement 472 form spatially distinct regions of the reflective surface473, the different portions of the radiation beam B correspond todifferent parts of an intensity distribution of the radiation beam B.

The surface elements may each have a width of the order of 1 to 100 μm.

Each branch radiation beam may comprise a plurality of sub-beams, eachreflected from a different surface element from a single group. Sinceeach of the surface elements within a given group of surface elements issubstantially parallel, each of the sub beams is substantially parallel,at least in the near field of the optical element 472. The surfaceelements from a given group are disposed at a non-zero angle to those ofother groups, i.e. surface elements from different groups are notsubstantially parallel. Therefore, in a near field of the opticalelement 472, the power distribution of each branch radiation beam willbe similar to that of the radiation beam B except there will be aplurality of strips, corresponding to the surface elements of othergroups of surface element, where the power distribution is substantiallyzero. However, due to a non-zero divergence of the radiation beam B, ina far field of the optical element 472, the plurality of sub beams willoverlap and will interfere to form a power distribution that issubstantially similar in shape to the radiation beam B.

Alternatively, the plurality of sub-beams from different surfaceelements may spread out sufficiently to interfere with each other in thefar field and each branch radiation beam may correspond to a localmaximum in an interference pattern from this interference.

Referring again to FIG. 25, in one embodiment, the geometry of thereflective surface 473 of the optical element 472 is such that first andsecond branch radiation beams B₁ B₂ are formed. Further, the geometry ofthe reflective surface 473 of the optical element 472 is such that apower of the first branch radiation beam B₁ is significantly smallerthan that of the second branch radiation beam B₂. The first branchradiation beam B₁ is directed to a sensor which is operable to determinea power and/or a power intensity distribution of the first radiationbeam B₁. The second branch radiation beam B₂ may, for example, bedirected to one or more of the lithographic apparatuses LA_(a)-LA_(n),which may be disposed in a far field of the optical element 472. Forsuch embodiments, an angular width of the surface elements that form thefirst branch radiation beam B₁ is preferably sufficiently small that, inthe far field, the plurality of strips in the power distributioncorresponding to these surface elements have been smoothed out bydiffraction due to the divergence of the second branch radiation beamB₂.

Optionally, a second optical element 474 may be provided to direct thesecond branch radiation beam B₂. The second optical element 474 may bearranged to ensure that the second branch radiation beam B₂ issubstantially parallel to the radiation beam B. In some embodiments,further optical elements may be provided to ensure that the secondbranch radiation beam B₂ is substantially aligned with the radiationbeam B.

In some embodiments, the geometry of the reflective surface 473 of theoptical element 472 is such that more than two branch radiation beamsare formed. For such embodiments, the geometry of the reflective surface473 of the optical element 472 may be such that a power of the firstbranch radiation beam B₁ is significantly smaller than that of theremaining branch radiation beams, which may have substantially equalpower. With such an arrangement, the apparatus for determining a valueindicative of the power of a radiation beam is combined with a beamsplitting apparatus that is arranged to split the beam into a pluralityof secondary beams. For such embodiments, the optical element may formpart of the beam splitting apparatus of FIG. 1.

An alternative embodiment of a sensor apparatus 480 for determining avalue indicative of a power of a radiation beam is shown in FIG. 27.Features of sensor apparatus 480 that directly correspond to sensorapparatus 470 have the same labels. Only the differences between sensorapparatus 480 and sensor apparatus 470 will be described in detail here.In this alternative embodiment, the sensor apparatus 480 comprises asensor 481 and an optical element 486 for receiving a radiation beam B.

The optical element 486 is a mirror, which may be a grazing incidencemirror. As with previous embodiments, the optical element is areflective grating arranged to produce a plurality of branch radiationbeams (in the example embodiment of FIG. 28, three). The second andthird branch radiation beams B₂, B₃ each comprise a plurality ofsub-beams, each reflected from a different surface element from a singlegroup of surface elements.

In addition, there will be a first portion of the radiation beam whichis not reflected by the surface elements so as to form part of thesecond or third branch radiation beams B₂, B₃. This scattered radiationmay predominantly comprise radiation which is incident upon edges formedat the intersections of adjacent surface elements. This scatteredradiation may cover a substantial solid angle and may be considered toform a first branch radiation beam B₁.

The sensor apparatus 480 further comprises a near-normal incidenceradiation collector 482 which is arranged to collect the first radiationbeam B₁ and direct it towards a sensor 481. The collector 482 isprovided with two apertures 483, 485 to allow the second and thirdbranch radiation beams B₂, B₃ to propagate away from the optical element486.

Such an arrangement conveniently uses the small fraction of unavoidablescattered radiation which would not contribute to the radiation receivedby the lithographic apparatuses LAa-LAn. Further, only a relativelysmall fraction of the incident radiation beam B is scattered in thisway, advantageously avoiding excessive heat loading of the sensor. Todetermine an intensity distribution of the radiation beam B, the sensorapparatus 480 is calibrated to ensure that the relationship between thepower and power distribution of the first branch radiation beam B₁ andthat of the radiation beam B is known.

Sensor apparatuses according to embodiments may form part of a beamsteering unit. In particular, measurements indicative of a powerdistribution of a radiation beam B made by sensor apparatuses accordingto embodiments of the invention may provide an input for afeedback-based control loop that is used to steer the radiation beam B.In response to the input, a direction of the radiation beam B may bealtered. This may be achieved, for example, by moving one or moreoptical elements in the path of the radiation beam B. Additionally oralternatively, this may be achieved by altering a trajectory of thebunched electron beam E in the free electron laser FEL. As a result ofthe optical guiding effect, the direction of the primary radiation beamB output by the free electron laser FEL will be dependent upon thetrajectory of the electron beam, especially within an end portion of theundulator 24.

The far field power distribution of a radiation beam produced by a freeelectron laser is expected to be Gaussian-like but to deviate from atrue Gaussian distribution. Sensor apparatuses according to embodimentsare particularly well suited providing an input for a feedback-basedcontrol loop that is used to steer a radiation beam B with such anunknown intensity distribution since that allow the radiation beamprofile to be sampled across the beam profile and, in particular, nearto the maximum of the beam power distribution. Further, the plurality ofmarks that form the first region of the optical element can besufficiently dense to allow the power and intensity distributiondirected to each lithographic apparatus LA_(a)-LA_(n) to be determinedby interpolation.

A free electron laser that is operable to supply radiation to aplurality of EUV lithographic apparatuses may, for example, have a powerof the order of tens of kilowatts and a diameter of the order of 100 μmat the output of the undulator 24, i.e. an average power density of theorder of GW/cm². Further, free electron laser radiation beam may havepulse lengths of the order of 100 fs or less, which can give rise topeak power densities of the order of 10¹⁴ W/cm². One way to measure thepower and/or position of such a radiation beam may be to place sensorson the periphery of the radiation beam profile. However, with such highpeak power intensities, the sensors would need to be placed severalsigma from the peak of the distribution. Therefore, such an arrangementwill not yield information regarding the intensity distribution of thetotal power. Further, such an arrangement would be very sensitive topointing instabilities of the free electron laser beam.

Referring again to FIGS. 1 and 2, the lithographic system LS may includeattenuators 15 a-15 n. The branch radiation beams B_(a)-B_(n) aredirected through a respective attenuator 15 a-15 n. Each attenuator 15a-15 n is arranged to adjust the intensity of a respective branchradiation beam B_(a)-B_(n) before the branch radiation beam B_(a)-B_(n)passes into the illumination system IL of its corresponding lithographicapparatus LA_(n)-LA_(n).

Referring to FIGS. 28a and 28b there is illustrated an example of anattenuation apparatus 519 that may correspond with the attenuator 15 ashown in FIGS. 1 and 2. The branch laser beam B_(a) is depicted by adashed-dot line. The attenuator 15 a comprises a first mirror 520 and asecond mirror 521. The second mirror 521 is separated, in a depictedy-direction, from the first mirror 520 by a distance 2 h. The secondmirror 521 is arranged so that the branch radiation beam B_(a) enteringthe attenuator 15 a is incident on a reflective surface of the firstmirror 520 and reflected by the reflective surface towards a reflectivesurface of the second mirror 521. The second mirror 521 is angled so asto direct the branch radiation beam B_(a) towards the lithographicapparatus LA_(a) (not shown in FIG. 28a ).

The first mirror 520 is connected to a first pivot point 522 via an arm520′, while the second mirror is connected to a second pivot point 523via an arm 521′. A first actuator (not shown) is provided to rotateabout the first pivot point 522, and a second actuator (not shown) isprovided to rotate the second mirror 521 around the second pivot point523. The positions of the mirrors 520, 521 are controlled by acontroller CTA. The first and second actuators may take any appropriateform as will be readily apparent to the skilled person. For example, theactuators may comprise motors disposed at the pivot points 522, 523 andconnected to the arms 520′, 521′.

Through rotation of the mirrors 520, 521 about the pivot points 522,523, an angle of incidence α of the mirrors 520, 521 with respect to thebranch radiation beam B_(a) may be adjusted. It will be appreciated thatas the mirrors 520, 521 are disposed at the same angle of incidence α,after reflection by the mirrors 520, 521, the branch radiation beamB_(a) propagates in the same direction as before reflection by themirrors 520, 521.

The mirrors 520, 521 are arranged to reflect the branch radiation beamB_(a) with what is commonly referred to as grazing (or glancing)incidence reflection. In FIG. 28a , the mirrors 520, 521 are showndisposed at a maximum angle of incidence α, such that the branchradiation beam is incident on a bottom portion (with respect to they-direction) of the mirror 520 and a top portion (with respect to they-direction) of the mirror 521. In some embodiments, the maximum valueof the angle α may be, for example, an angle of approximately 10degrees.

In FIG. 28b , the mirrors 520, 521 are shown disposed at a minimum angleα of incidence such that the branch radiation beam B_(a) is incident ona top portion of the mirror 520 and a bottom portion of the mirror 521.The minimum value of the angle α may be, for example, an angle α ofapproximately 1 degrees. In the depicted example, therefore, the mirrors520, 521 are rotatable about the respective pivot points 522, 523between angles of incidence of 1 degrees to 10 degrees. It will beappreciated that in other embodiments, the arrangement and/or size ofmirrors 520, 521 may be different so as to allow a larger or smallerangular range. For example, the pivot points 522, 523 may be selected soas to increase or decrease the useful angular range of the mirrors 520,521. Further, while the mirrors 520, 521 are each shown as beingarranged to rotate around a fixed pivot point, this is merely exemplary.It will be appreciated that the angle of incidence of the mirrors 520,521 may be adjusted using any other arrangement as will be readilyapparent to the skilled person. In an embodiment, the mirrors 520, 521may both be arranged to rotate about the same pivot point. Byappropriate selection of the position of the pivot points 522, 523, adisplacement of the outgoing branch radiation beam B_(a) with respect tothe incoming branch radiation beam B_(a), (i.e. 2 h in the embodiment ofFIGS. 28 a, 28 b), can be made substantially constant for angles αwithin a predetermined, relatively small range (as shown in FIGS. 28a,28b ). For larger angular ranges of the angle α, however, where thedisplacement of the outgoing branch radiation beam with respect to theincoming branch radiation beam is to be substantially constant, at leastone of the mirrors 520, 521 or both, may be provided with translationalmeans suitable to translate one or both of the mirrors 520, 521 in they-direction.

The reflectance of each of the mirrors 520, 521 is a function of theangle of incidence α between the mirror 520, 521 and the branchradiation beam B_(a). For example, for an incidence angle of 2 degrees,approximately 98% (in a theoretical case of a mirror having a ruthenium(Ru) coating having perfectly flat surface) of the incident radiationmay be reflected at each of the mirrors 520, 521. That is, when angledat 2 degrees, radiation reflected by one of the mirrors 520, 521 isreduced by 2% compared to the intensity of the radiation that isincident on that mirror. As such, where both of the mirrors 520, 521 aredisposed at an angle α of 2 degrees, the intensity of the branchradiation beam B_(a) is reduced by approximately 4% through reflectionby the mirrors 520, 521.

For an incidence angle of 10 degrees (the maximum angle used in theexample above), approximately 90% of the incident radiation may bereflected at each of the mirrors 520, 521. That is, when the angle ofincidence is 10 degrees, the intensity of the reflected radiation isapproximately 10% less than the incident radiation. As such, where bothof the mirrors 520, 521 are disposed at an angle of incidence α of 10degrees, the intensity of the branch radiation B_(a) is reduced byapproximately 20% through reflection by the mirrors 520, 521.

From the above description, it will be appreciated that by adjustment ofthe angle α between 1 and 10 degrees, the intensity of the branchradiation beam B_(a) received at the lithographic apparatus LA_(a) maybe varied between 2% and 20%.

In some embodiments the angle of incidence of the mirrors 520, 521 maybe adjusted at a frequency of up to 1 KHz, thereby providing anadjustment mechanism for the attenuation of the branch laser beam B_(a).The first and second actuators (e.g. motors) may be connected to acontroller CTA. The controller CTA may be arranged to receiveinstructions indicating a desired intensity of the branch radiation beamB_(a) to be received at the lithographic apparatus LA_(a). In responseto receipt of such instructions, the controller may be arranged tocontrol the actuator to adjust the angle of incidence α of the mirrors520, 521 to achieve a desired attenuation of the branch radiation beamB_(a) and thereby a desired intensity at the lithographic apparatusLA_(a). The controller may receive as input from a sensor SL_(a) (seeFIG. 2) a measurement indicative of the intensity of the branchradiation beam B_(a) in the lithographic apparatus LA_(a).

The controller CTA may be part of a feedback-based control loop arrangedto detect an intensity of the branch radiation beam B_(a) at thelithographic apparatus LA_(a) and to adjust the attenuation of thebranch radiation beam B_(a) in order to maintain the intensity at thelithographic apparatus LA_(a) at a predetermined value or within apredetermined range. Referring to FIG. 1, this feedback-based controlloop F2 _(a) may be separate from a feedback-based control loop F1provided after the free electron laser and before the beam splitter 19.The feedback-based control loop F2 _(a) which controls the attenuator 15a may be referred to as a second feedback-based control loop. Thefeedback-based control loop F1 provided after the free electron laserand before the beam splitter 19 may be referred to as a firstfeedback-based control loop. The first and second feedback-based controlloops F1, F2 _(a) may be operated independently of each other. They maybe controlled by different controllers CT, CTA or may be controlled bythe same controller. The second feedback-based control loop F2 _(a) maybe slower than the first feedback-based control loop F1.

In other embodiments, the angles of incidence of each of the mirrors520, 521 may be adjustable independently of one another. While thiswould result in a change in the direction of propagation of the branchradiation beam B_(a), this may beneficially increase the number possibleattenuation values in, for example, embodiments in which the angle ofincidence of a mirror 520, 521 is adjustable only in discrete steps.

It will be appreciated that while the embodiments described above aredescribed with reference to the attenuator 15 a, the attenuators 15 b-15n may be similarly implemented.

Referring to FIG. 29, there is illustrated an alternative embodiment ofan attenuation apparatus 519 that may comprise the attenuator 15 a. Inthe embodiment of FIG. 29, the attenuation apparatus 519 comprises fourmirrors 530, 531, 532, 533. The mirrors 530, 531 are arranged similarlyto the mirrors 520, 521 as described above with reference to FIGS. 28a,28b . In particular, the first mirror 530 is provided with a firstactuator arranged to rotate the mirror 530 about a first pivot point 534to which the mirror 530 connects via an arm 530′. The second mirror 531is provided with a second actuator arranged to rotate the mirror 531about a second pivot point 535 to which the mirror 531 connects via anarm 531′.

The mirrors 532, 533 are arranged similarly to the mirrors 530, 531, butmay be considered to be a “mirroring” of the arrangement of the firstmirror 530 and the second mirror 531 along a an axis perpendicular tothe direction propagation of the branch radiation beam B_(a). Inparticular, the third mirror 532 is disposed at the same position in they-direction as the second mirror 531 and is arranged to receiveradiation reflected from the second mirror 531. The third mirror isprovided with a third actuator arranged to rotate the mirror 532 about athird pivot point 536. The third mirror 532 is arranged to reflectreceived radiation towards the fourth mirror 533 which is separated fromthe second mirror 532 in the y-direction by a distance of 2 h (i.e. thefourth mirror 553 is at the same position in the y-direction as thefirst mirror 530). The fourth mirror 553 is provided with a fourthactuator arranged to rotate the mirror 553 about a fourth pivot point537. The fourth mirror 553 is arranged to direct radiation to thelithographic apparatus LA_(a) (not shown in FIG. 29).

Where the angle of incidence α of each of the first to fourth mirrors530-553 is the same, the branch radiation beam B_(a) exits theattenuator 15 a in the same direction and at the same position in they-direction as it enters the attenuator 15 a. Additionally, by usingfour mirrors, each being operable to adjust the angle of incidencethrough a range of 1 degrees and 10 degrees, a possible attenuationrange of the attenuator 15 a is increased from a range of 2% to 20% (inthe arrangement of FIG. 28) to a range of 4% to 40% (i.e. a possibletransmission range of 96% to 60% of the radiation entering theattenuator 15 a). It will be appreciated that where a greater minimumattenuation is acceptable, the greater range of attenuation achievablein the embodiment of FIG. 29 may be advantageous.

Further, the embodiment of FIG. 29 may be utilised to provide the sameor a similar attenuation range to that which may be provided by theembodiment of FIG. 28 with a smaller effect on the polarization of thebranch radiation beam B_(a). That is, due to the smaller angle ofincidence α required to achieve a particular attenuation. The combinedeffect of the four mirrors 530 to 553 on the P and S polarizationcomponents of the branch radiation beam B_(a) is smaller than thecombined effect of the two mirrors 520, 521 for a given attenuation.This is particularly the case for attenuations of or approaching 20%(i.e. as the angle of incidence α of each mirror 520, 521 approaches 10degrees).

In some embodiments it may be desired to retain, as far as possible, agenerally circular polarization exhibited by the branch radiation beamB_(a) before it enters the attenuator 15 a. In this case, an attenuationrange of approximately 2% to 20% may be achieved with an angularadjustment range of between approximately 1 degrees and 5 degrees. Thisembodiment may therefore be particularly beneficial for having a reducedeffect on the polarization of the branch radiation beam B_(a).

Further, in the arrangement of FIG. 29, translational means forproviding translational correction of one or more of the mirrors 530 to553 are not required. The outgoing beam has the same angle and positionas the incoming beam for all values of alpha (when angles alpha areequal for all four mirrors). Put another way, any change in the distance2 h caused by the mirrors 530, 531 is “reversed” by the mirrors 532,553, such that translation of the mirrors in the y-direction is notrequired to ensure that the branch radiation beam B_(a) leaves theattenuator 15 a at the same position as it enters.

FIG. 29 may be considered to show two sets of two mirrors; a first setcontaining the mirrors 530, 531 and a second set containing the mirrors532, 533. It will be appreciated that in other embodiments additionalmirrors, or additional sets of mirrors may be provided to furtherincrease the possible attenuation range, or to reduce alterations to thepolarization of the branch radiation beam B_(a).

One or more of the attenuators 15 a to 15 n may comprise an alternativeattenuation apparatus (e.g. in addition to or instead of the attenuationapparatus described above). The alternative attenuation apparatus mayprovide a fixed attenuation or may provide an adjustable attenuation.Where an adjustable attenuation is provided the adjustment may have aslower speed than the speed of the above described attenuationapparatus. The alternative attenuation apparatus may have a higher rangeof possible attenuation values.

FIG. 30a schematically depicts an example of an alternative attenuationapparatus 540 that may be provided in combination with, or in thealternative to, the above described attenuation apparatus. Where theseattenuation apparatus are provided in combination, the branch radiationbeam B_(a) may pass through either attenuation apparatus before passingthrough the other. The attenuation apparatus 540 is controlled by acontroller CTA.

The attenuation apparatus 540 is gas-based and comprises a housing 541defining a chamber 542. The housing 541 may define a chamber 542 of anyshape. For example, the housing 541 may be generally tubular. Thechamber 542 is closed at a first end by a first window 543 and at asecond, opposing end, by a second window 544. An inlet 545 is providedto allow a controlled amount of a gas, into the chamber 542. A valve 546may also be provided to allow a controlled flow of gas from the chamber542. A pressure monitor 547 is provided to monitor a pressure within thechamber 542. The pressure monitor 547 may be any form of pressuremonitor. By providing a gas flow, rather than a fixed, enclosed gasmedium, energy absorbed by the gas may be removed. The amount of energythus removed may be substantial where the attenuation apparatus 540provides a large attenuation factor (such as a factor of 10).

The inlet 545 allows the introduction into the chamber 542 of an EUVabsorbing gas. It will be appreciated that the particular gas introducedinto the chamber 542 may be selected in dependence upon a desired levelof EUV absorption. As an example, however, gasses such as Hydrogen,Helium and/or Argon may be suitable. The windows 543, 544 areconstructed so as to provide a high transmittance for EUV radiation andmay be constructed to provide a high absorbance to other wavelengths ofelectromagnetic radiation. For example, the windows may comprise whatare commonly referred to as spectral purity filters, which filterradiation outside of the EUV wavelength, but which allow thetransmission of EUV radiation. Such spectral purity filters may beconstructed in any appropriate way as will be apparent to those skilledin the art. For example, the windows 543, 544 may be constructed frommolybdenum (Mo) and zirconium silicide (ZrSi). The Mo/ZrSi stack may becapped on one or both sides with molybdenum silicide (MoSi). In analternative example the windows 543, 544 may be formed from polysilicon(pSi) One or both of the sides of the polysilicon film may be cappedwith a silicon nitride (SiN) layer. Other materials, for examplegraphene, may be suitable for use in the windows 543, 544. The thicknessof the windows 543, 544 may be selected in dependence upon a maximumpressure desired within the chamber 542, which itself may be selected independence upon a desired attenuation.

The branch radiation beam B_(a) enters the alternative attenuationapparatus 540 through the first window 543 and is attenuated by way ofinteraction with the fluid within the chamber 542, before exiting theattenuation apparatus 540 through the second window 544. An attenuationof the branch radiation beam B_(a) caused by passage through the chamber542 may be varied by varying the type, amount or pressure of gas withinthe chamber 542.

The pressure sensor, gas inlet and gas valve may be in communicationwith a controller CTA. The controller CTA may be operable to control thegas inlet 545 and the gas valve 546 to achieve a desired pressure withinthe chamber 542. The desired pressure within the chamber 542 may beselected so as to achieve a desired attenuation of the branch radiationbeam B_(a) to be caused by the alternative attenuation apparatus.Alternatively or additionally, a desired pressure within the chamber 542may be selected to maintain a pressure within the chamber 542 within apredetermined safe range.

An alternative embodiment of the alternative attenuation apparatus isillustrated in FIG. 30b in which like components have been provided withlike reference numerals. In the example embodiment of FIG. 30a , both ofthe windows 543, 544 are perpendicular to the direction of propagationof the branch radiation beam B_(a) along their length. As such, the pathof the branch radiation beam B_(a), through the chamber 542, is the samelength irrespective of the position at which the branch radiation beamB_(a) enters the chamber 542. In the alternative example shown in FIG.30b , the windows 543, 544 are angled towards each other with respect tothe direction of propagation of the branch radiation beam B_(a). In thisway, where the branch radiation beam B_(a) enters the chamber 542 at oneposition, it will travel a shorter distance through the chamber 542 thanwhen the branch radiation beam B_(a) enters the chamber 542 at adifferent, lower (in the y-direction in FIG. 30b ) position. As such,attenuation of the branch radiation beam can be varied by varying theposition at which branch radiation beam B_(a) enters the chamber 542.Moreover, this arrangement can also be used to generate an intensitygradient over the cross section of the light beam. Such an intensitygradient may be used to correct for intensity variations over theillumination field.

A further alternative gas-based attenuation apparatus 550 is shownschematically in FIGS. 31 and 32. Referring first to FIG. 31, theapparatus 550 comprises a tube 551 through which the branch radiationbeam B_(a) passes. Gas which acts to attenuate the branch radiation beamB_(a) is supplied from a gas supply 552 via three valves 553 a-c to gasinlets 554 a-c which are spaced apart along the tube 551. Three outlets555 a-c are provided in the tube 551, the gas outlets being generallyopposite associated gas inlets 554 a-c. A vacuum pump 556 a-c isconnected to each outlet 555 a-c and configured to pump gas to anexhaust 557.

The attenuation apparatus 550 further comprises differential pumpingsections, two of which 558 are provided upstream of the gas inlets 554a-c and two of which 559 are provided downstream of the gas inlets. Eachdifferential pumping section 558, 559 comprises a volume in the tube 551which is partially enclosed by walls 560. The walls 560 are eachprovided with an opening through which the branch radiation beam B_(a)travels. A pump 561 is connected to each volume and is used to pump gasfrom that volume. The differential pumping sections 558, 559 areoperable to isolate pressure fluctuations in the attenuation apparatus550 from other apparatus (e.g. to isolate them from a lithographicapparatus).

In use, the degree of attenuation provided by the attenuation apparatus550 is controlled by changing the gas flow rate of gas passing throughthe valves 553 a-c. The valve 553 a-c may be controlled by a controllerCTA. The pressure of gas in the tube 551 may be increased to increaseattenuation of the branch radiation beam B_(a) and may be decreased toreduce attenuation of the branch radiation beam. The attenuation may bereduced to 0% by removing all of the gas from the tube 551 using thevacuum pumps 556 a-c. The attenuation provided by the gas will dependupon the length of tube 551 over which the gas is provided and inaddition will depend upon the gas that is used. For example, absorptionof EUV radiation is 0.1% per Pa per meter when hydrogen gas is used. Ifattenuation between 0% and 20% of the branch radiation beam B_(a) isneeded and the gas is provided over a tube length of 10 m, then thepressure of hydrogen gas in the tube should be varied between 0 Pa and20 Pa. If a gas with a higher absorption coefficient is used, forexample argon (absorption 0.034 Pa⁻¹ m⁻¹), nitrogen (0.059 Pa⁻¹ m⁻¹), orxenon (0.062 Pa⁻¹ m⁻¹) then the length of the tube 551 may be reducedaccordingly. For example, when using nitrogen attenuation between 0% and20% may be achieved using a tube 5 m long with a pressure range of 0-0.7Pa.

The response time of the attenuation apparatus 550 will depend on thespeed of valves 553 a-c, on the pumps speeds of the vacuum pumps 556a-c, and on the volume of the tube 551 to which gas is provided.

FIG. 32 shows schematically an embodiment of the valve 553 a and thepump 556 a. The valve 553 a comprises a baffle 570 which is actuated byan actuator 571. The baffle 570 forms a leaky seal between a buffervolume 572 and the tube 551. The buffer volume 572 is kept at a higherpressure than the pressure within the tube 551 using the gas supply 552.The actuator 571 moves the baffle 570 to open and close an outlet 554 afrom the buffer volume 572 into the tube 551. The baffle 570 may belightweight (e.g. weighing 10 g or less, for example around 1 g). As aresult the actuator 571 may be able to open and close the outlet 554 ausing the baffle 570 at a relatively high frequency (e.g. in excess of 2kHz).

The vacuum pump 556 a has a pump speed which may be expressed as alinear velocity v at which gas enters an inlet aperture of the pump. Theresponse time is roughly related to the diameter D of the tube 551(response time T is roughly D/v). For a typical turbo-molecular pump vis around 100 m/s. If the tube 551 has a diameter D of 5 cm, then thiswould provide a response time T of around 0.5 ms. This corresponds to amaximum frequency of around 2 kHz. More than one pump 556 a may beprovided around the tube 551 in the vicinity of the gas inlet 554 a.Where this is done the response frequency will be increased.

Generally, the range in which attenuation of the branch radiation beamB_(a) may be varied using the alternative attenuation apparatus of FIGS.30a, 30b is larger than the range of attenuation adjustment achievablewith the attenuation apparatus of FIGS. 28 and 29. However, the speedwith which the attenuation may be adjusted is slower. For example, thechamber 542 may be emptied of gas in order to decrease the attenuation.However, this may take a significant length of time compared to the timerequired to adjust the mirrors 530 to 553, for example. The length oftime may be longer than the time period during which a target locationreceives EUV radiation (e.g. longer than 1 ms).

Referring to FIG. 33, there is shown a further alternative embodiment,in which an attenuation apparatus is provided by an EUV reflectivemembrane 580 disposed in the path of the branch radiation beam B_(a) ata near-normal angle of incidence. The membrane 580 may be constructedsimilarly to the windows 543, 544 described above. The membrane 580 maybe of any suitable dimensions depending on the construction andmaterials used.

The branch radiation beam B_(a) leaves the first attenuation apparatus519 and is incident upon the membrane 580. The membrane 580 is orientedso as to create an angle of incidence of the branch radiation beam B_(a)which causes a portion 581 of the branch radiation beam B_(a) to bereflected towards a radiation dump 582 disposed on a wall of theattenuator 515 a. A portion 553 of the branch radiation beam B_(a) istransmitted through the membrane 580. It will be appreciated that aportion of the branch radiation beam B_(a) not reflected will beabsorbed by the membrane 580. The angle of incidence of the branchradiation beam B_(a) and the membrane 580 may be a near-normal incidenceangle substantially avoiding reflection radiation towards a previousoptical element (e.g. an attenuation apparatus 519 in FIG. 33).

The membrane 580 may be moved between a first position (not shown) inwhich it does not intersect with the branch radiation beam Ba and asecond position (shown) in which it does intersect with the radiationbeam. The position of the membrane may be controlled by a controller CTAusing an actuator (not shown). The controller CTA thus selects betweenthe first position and the second position depending upon whether or notit is desired to provide attenuation using the membrane 580.

In FIG. 33, the membrane 580 is disposed after the attenuation apparatus519 (with respect to the direction of propagation of the branchradiation beam B_(a)) within the attenuator 15 a. However in otherembodiments, the order of attenuation apparatus within the attenuator 15a may be otherwise. It will further be appreciated that a plurality ofmembranes such as the membrane 580 may be provided in sequence tofurther increase an attenuation of the branch radiation beam B_(a).Intersection of the plurality of membranes with the branch radiationbeam may be controlled by the controller CTA.

In an embodiment, a mesh may be used instead of the membrane 580. In anembodiment, two or more meshes may be used. A mesh may be capable ofwithstanding a higher thermal load than a membrane.

In an embodiment an attenuator may comprise an adjustable aperture atthe opening 8 of the enclosing structure of the illumination system IL(see FIG. 2). The size of the adjustable aperture may be reduced inorder to attenuate the branch radiation beam Ba. This embodiment doesnot affect the far-field distribution of the radiation. It has only aminor impact on the radiation in the pupil plane (poles of radiationwill become smaller but will not change position).

An attenuator 15 a-n may comprise one or more of the above describedembodiments. For example, the reflective membrane of FIG. 33 may becombined with the attenuation apparatus of FIG. 28 or 29 and/or theattenuation apparatus of FIG. 30a, 30b Other combinations of embodimentsare also possible.

While it is described above in connection with FIG. 1 that a respectiveattenuator 15 a-15 n is provided for each branch radiation beam, it willbe appreciated that in other embodiments, an attenuator may be providedfor only one or some of the branch radiation beams. Further, a singleattenuator may be provided for a plurality of branch radiation beams.For example, although the attenuators 15 a-15 n are shown disposedoutside of the splitter 19, in other embodiments, an attenuator asdescribed herein may be disposed within the splitter 19 so as toattenuate a plurality of branch radiation beams. For example, toattenuate all of the branch radiation beams B_(b)-B_(n) together, anattenuator may be provided immediately after the branching of the firstbranch radiation beam B_(a). Any combination or configuration ofattenuators may be provided.

An attenuator as generally described above may be positioned elsewherewithin the lithographic system before the substrate. For example, withreference to FIG. 2, an attenuator may be positioned within theillumination system IL.

Whilst embodiments of the invention have been described in the contextof a single free electron laser FEL, it should be appreciated that anynumber of free electron lasers FEL may be used. For example, two freeelectron lasers may be arranged to provide EUV radiation to a pluralityof lithographic apparatus. This is to allow for some redundancy. Thismay allow one free electron laser to be used when the other freeelectron laser is being repaired or undergoing maintenance.

Although the described embodiments of a lithographic system LS may referto eight lithographic apparatuses, a lithographic system LS may compriseany number of lithographic apparatus. The number of lithographicapparatus which form a lithographic system LS may, for example, dependon the amount of radiation which is output from a free electron laserand on the amount of radiation which is lost in a beam splittingapparatus 19. The number of lithographic apparatus which form alithographic system LS may additionally or alternatively depend on thelayout of a lithographic system LS and/or the layout of a plurality oflithographic systems LS.

Embodiments of a lithographic system LS may also include one or moremask inspection apparatus MIA and/or one or more Aerial InspectionMeasurement Systems (AIMS). In some embodiments, the lithographic systemLS may comprise two mask inspection apparatuses to allow for someredundancy. This may allow one mask inspection apparatus to be used whenthe other mask inspection apparatus is being repaired or undergoingmaintenance. Thus, one mask inspection apparatus is always available foruse. A mask inspection apparatus may use a lower power radiation beamthan a lithographic apparatus. Further, it will be appreciated thatradiation generated using a free electron laser FEL of the typedescribed herein may be used for applications other than lithography orlithography related applications.

The term “relativistic electrons” should be interpreted to meanelectrons which have relativistic energies. An electron may beconsidered to have a relativistic energy when its kinetic energy iscomparable to or greater than its rest mass energy (511 keV in naturalunits). In practice a particle accelerator which forms part of a freeelectron laser may accelerate electrons to energies which are muchgreater than its rest mass energy. For example a particle acceleratormay accelerate electrons to energies of >10 MeV, >100 MeV, >1 GeV ormore.

Embodiments of the invention have been described in the context of afree electron laser FEL which outputs an EUV radiation beam. However afree electron laser FEL may be configured to output radiation having anywavelength. Some embodiments of the invention may therefore comprise afree electron which outputs a radiation beam which is not an EUVradiation beam.

The term “EUV radiation” may be considered to encompass electromagneticradiation having a wavelength within the range of 4-20 nm, for examplewithin the range of 13-14 nm. EUV radiation may have a wavelength ofless than 10 nm, for example within the range of 4-10 nm such as 6.7 nmor 6.8 nm.

The lithographic apparatuses LA_(a) to LA_(n) may be used in themanufacture of ICs. Alternatively, the lithographic apparatuses LA_(a)to LA_(n) described herein may have other applications. Possible otherapplications include the manufacture of integrated optical systems,guidance and detection patterns for magnetic domain memories, flat-paneldisplays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

Different embodiments may be combined with each other. Features ofembodiments may be combined with features of other embodiments.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

The invention claimed is:
 1. A method of patterning lithographicsubstrates, the method comprising using a free electron laser togenerate EUV radiation and delivering the EUV radiation to alithographic apparatus which projects the EUV radiation ontolithographic substrates, wherein the method further comprises reducingfluctuations in the power of EUV radiation delivered to the lithographicsubstrates by using a feedback-based control loop to monitor the freeelectron laser and adjust operation of the free electron laseraccordingly, and wherein the method further comprises applying variableattenuation to EUV radiation that has been output by the free electronlaser in order to further control the power of EUV radiation deliveredto the lithographic apparatus.
 2. The method of claim 1, wherein thelithographic apparatus is one of a plurality of lithographic apparatuseswhich receives the EUV radiation.
 3. The method of claim 2, wherein thevariable attenuation of the EUV radiation is independently controllablefor each of the lithographic apparatuses.
 4. The method of claim 1,wherein the variable attenuation is controlled by a secondfeedback-based control loop.
 5. The method of claim 4, wherein thesecond feedback-based control loop operates at a frequency of 1 kHz orless.
 6. The method of claim 4, wherein the second feedback-basedcontrol loop uses EUV radiation intensity as measured by a sensorlocated in the lithographic apparatus, the sensor being located before aprojection system of the lithographic apparatus.
 7. The method of claim4, wherein the second feedback-based control loop uses EUV radiationintensity as measured by a sensor located in the lithographic apparatus,the sensor being located after a projection system of the lithographicapparatus.
 8. A lithographic system comprising a free electron laserconfigured to generate EUV radiation and a lithographic apparatusconfigured to project the EUV radiation onto lithographic substrates,wherein the apparatus further comprises a feedback-based control loopcomprising a sensor configured to monitor the EUV radiation output bythe free electron laser and a controller configured to receive an outputfrom the sensor and to adjust operation of the free electron laseraccordingly, and wherein the apparatus further comprises an attenuatorconfigured to apply variable attenuation to EUV radiation that has beenoutput by the free electron laser in order to further control the powerof EUV radiation delivered to the lithographic apparatus.
 9. Thelithographic system of claim 8, wherein the lithographic apparatus isone of a plurality of lithographic apparatuses which receives the EUVradiation.
 10. The lithographic system of claim 9, wherein an attenuatoris provided for each lithographic apparatus, the attenuators beingindependently controllable for each of the lithographic apparatuses. 11.The lithographic system of claim 8, wherein the variable attenuation iscontrolled by a second feedback-based control loop.
 12. The lithographicsystem of claim 11, wherein the second feedback-based control loopoperates at a frequency of 1 kHz or less.
 13. The lithographic system ofclaim 11, wherein the second feedback-based control loop comprises asensor configured to measure EUV radiation intensity in the lithographicapparatus.
 14. The lithographic system of claim 13, wherein the sensoris located before a projection system of the lithographic apparatus. 15.The lithographic system of claim 13, wherein the sensor is located aftera projection system of the lithographic apparatus.